Skip sha256 repo tests if sha256 not supported (#13018)

Otherwise the newly introduced test from #12335 fails on platforms which do not have git sha256 yet

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### Tests for Go 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

### 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.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/13018
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>

modules/git/repo_test.go

@@ -148,6 +148,11 @@ func TestInitRepositoryWithNoTemplates(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			if !IsValidObjectFormat(tt.objectFormatName) {
+				t.Skip("not supported")
+				return
+			}
+
 			repoPath := t.TempDir()
 
 			err := InitRepository(t.Context(), repoPath, tt.bare, tt.objectFormatName)