repository: Don't use seeded repository in GarbageCollect test

Stop using seeded repositories in one of the tests for the
GarbageCollect RPC. While at it, parallelize the subtests. Together this
significantly speeds up the test:

Before: `✓  internal/gitaly/service/repository (2.578s)`
After:  `✓  internal/gitaly/service/repository (1.146s)`

1 files changed, 9 insertions, 6 deletions

diff --git a/internal/gitaly/service/repository/gc_test.go b/internal/gitaly/service/repository/gc_test.go
index 28572124b..00a0e19e3 100644
--- a/internal/gitaly/service/repository/gc_test.go
+++ b/internal/gitaly/service/repository/gc_test.go
@@ -207,7 +207,7 @@ func TestGarbageCollectDeletesPackedRefsLock(t *testing.T) {
 	ctx := testhelper.Context(t)
 	cfg, client := setupRepositoryServiceWithoutRepo(t)
 
-	testCases := []struct {
+	for _, tc := range []struct {
 		desc        string
 		createLock  func(t *testing.T, lockfilePath string)
 		shouldExist bool
@@ -230,13 +230,16 @@ func TestGarbageCollectDeletesPackedRefsLock(t *testing.T) {
 			desc:        "with a non-existing lock",
 			shouldExist: false,
 		},
-	}
+	} {
+		tc := tc
 
-	for _, tc := range testCases {
 		t.Run(tc.desc, func(t *testing.T) {
-			repo, repoPath := gittest.CreateRepository(t, ctx, cfg, gittest.CreateRepositoryConfig{
-				Seed: gittest.SeedGitLabTest,
-			})
+			t.Parallel()
+
+			repo, repoPath := gittest.CreateRepository(t, ctx, cfg)
+
+			gittest.WriteCommit(t, cfg, repoPath, gittest.WithBranch("main"))
+			gittest.Exec(t, cfg, "-C", repoPath, "pack-refs", "--all")
 
 			// Force the packed-refs file to have an old time to test that even
 			// in that case it doesn't get deleted