test: fix test that was supposed to test DST behaviour but did not (#12007)

https://codeberg.org/forgejo/forgejo/pulls/11851 introduced tests that verify the scheduling of Forgejo Actions workflows during daylight saving time (DST) changes. Unfortunately, one test didn't test what it was supposed to because it used a reference time in UTC that was already after the clock change has happened.

This change also adds tests that verify that `NewActionScheduleSpec()` respects time zones when calculating the initial execution time of a scheduled workflow.

## Checklist

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### Tests for Go changes

(can be removed for JavaScript changes)

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I ran...
  - [x] `make pr-go` before pushing

### Tests for JavaScript changes

(can be removed for Go changes)

- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
- [x] This change is not visible to a Forgejo user or admin (refactor, dependency upgrade, etc.). I think there is no need to add a release note for this change.

*The decision if the pull request will be shown in the release notes is up to the mergers / release team.*

The content of the `release-notes/<pull request number>.md` file will serve as the basis for the release notes. If the file does not exist, the title of the pull request will be used instead.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12007
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
Co-authored-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>
Co-committed-by: Andreas Ahlenstorf <andreas@ahlenstorf.ch>

models/actions/schedule_spec_test.go

@@ -13,6 +13,44 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+func TestActionScheduleSpec_NewActionScheduleSpec(t *testing.T) {
+	tests := []struct {
+		name        string
+		refTime     time.Time
+		cronPattern string
+		timeZone    string
+		want        string
+		wantErr     assert.ErrorAssertionFunc
+	}{
+		{
+			name:        "without timezone",
+			refTime:     time.Date(2026, 4, 6, 11, 56, 0, 0, time.UTC),
+			cronPattern: "58 14 * * *",
+			want:        "2026-04-06T14:58:00Z",
+			wantErr:     assert.NoError,
+		},
+		{
+			name:        "with separate timezone",
+			refTime:     time.Date(2026, 4, 6, 11, 56, 0, 0, time.UTC),
+			cronPattern: "58 14 * * *",
+			timeZone:    "Europe/Tallinn", // +03 (EEST)
+			want:        "2026-04-06T11:58:00Z",
+			wantErr:     assert.NoError,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			s, err := NewActionScheduleSpec(test.cronPattern, optional.FromNonDefault(test.timeZone), test.refTime)
+			test.wantErr(t, err)
+
+			if err == nil {
+				assert.Equal(t, test.want, s.Next.AsTime().UTC().Format(time.RFC3339))
+			}
+		})
+	}
+}
+
 func TestActionScheduleSpec_Parse(t *testing.T) {
 	// Mock the local timezone is not UTC
 	local := time.Local
@@ -88,24 +126,24 @@ func TestActionScheduleSpec_Parse(t *testing.T) {
 			// skipping the execution on that day because any time between 2 and 3 o'clock never happened. Forgejo uses
 			// option B because the code it inherited already did that and was exposed to users.
 			name:     "skips execution during DST jump forwards",
-			refTime:  time.Date(2025, 3, 30, 1, 5, 0, 0, time.UTC),
-			spec:     "10 2 * * *", // The clock jumps at 2 o'clock to 3 o'clock.
+			refTime:  time.Date(2025, 3, 30, 0, 55, 0, 0, time.UTC), // 01:55 local time
+			spec:     "10 2 * * *",                                  // The clock jumps at 2 o'clock to 3 o'clock.
 			timeZone: "Europe/Berlin",
 			want:     "2025-03-31T00:10:00Z",
 			wantErr:  assert.NoError,
 		},
 		{
 			name:     "executes a first time before DST jump backwards",
-			refTime:  time.Date(2025, 10, 26, 0, 5, 0, 0, time.UTC),
-			spec:     "10 2 * * *", // The clock jumps at 3 o'clock to 2 o'clock.
+			refTime:  time.Date(2025, 10, 26, 0, 5, 0, 0, time.UTC), // 02:05 local time
+			spec:     "10 2 * * *",                                  // The clock jumps at 3 o'clock to 2 o'clock.
 			timeZone: "Europe/Berlin",
 			want:     "2025-10-26T00:10:00Z",
 			wantErr:  assert.NoError,
 		},
 		{
 			name:     "executes a second time after DST jump backwards",
-			refTime:  time.Date(2025, 10, 26, 1, 5, 0, 0, time.UTC),
-			spec:     "10 2 * * *", // The clock jumps at 3 o'clock to 2 o'clock.
+			refTime:  time.Date(2025, 10, 26, 1, 5, 0, 0, time.UTC), // 02:05 local time
+			spec:     "10 2 * * *",                                  // The clock jumps at 3 o'clock to 2 o'clock.
 			timeZone: "Europe/Berlin",
 			want:     "2025-10-26T01:10:00Z",
 			wantErr:  assert.NoError,