fix(sync): eliminate periodic sync race in TestDockerImagesAreSkipped

The test only exercises on-demand sync but had `PollInterval: 30m` set
with non-empty `Content`, causing the scheduler to immediately start
a background periodic sync goroutine on server startup. This goroutine
created temporary `.sync/<uuid>` directories inside `destDir/zot-test`
concurrently with the test's `os.RemoveAll(destDir/testImage)`, causing
an intermittent "directory not empty" failure.

Fix: remove `PollInterval` from the sync registry config so the periodic
sync generator is never registered with the scheduler. On-demand sync
(the only mode exercised by this test) is unaffected.

Agent-Logs-Url: https://github.com/project-zot/zot/sessions/1a321663-3147-46e9-9321-989e5dd0ed3c

Co-authored-by: rchincha <45800463+rchincha@users.noreply.github.com>

pkg/extensions/sync/sync_test.go

@@ -1417,8 +1417,6 @@ func TestDockerImagesAreSkipped(t *testing.T) {
 
 	for _, testCase := range testCases {
 		Convey("Verify docker images are skipped when they are already synced, preserveDigest: "+testCase.name, t, func() {
-			updateDuration, _ := time.ParseDuration("30m")
-
 			sctlr, srcBaseURL, srcDir, _ := makeUpstreamServer(t, false, false)
 
 			scm := test.NewControllerManager(sctlr)
@@ -1443,7 +1441,6 @@ func TestDockerImagesAreSkipped(t *testing.T) {
 					},
 				},
 				URLs:           []string{srcBaseURL},
-				PollInterval:   updateDuration,
 				TLSVerify:      &tlsVerify,
 				CertDir:        "",
 				MaxRetries:     &maxRetries,