Add cache test for admins (#31265)

Add a test to probe the cache similar to the email test func.


![image](https://github.com/go-gitea/gitea/assets/24977596/700e2733-586d-4091-900f-f5f71e6e94bf)


![image](https://github.com/go-gitea/gitea/assets/24977596/2a953802-18fc-4e81-a37d-24ebe1297365)


![image](https://github.com/go-gitea/gitea/assets/24977596/e00d62ad-bb60-41cc-9138-09993daee156)

---------

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: silverwind <me@silverwind.io>

modules/cache/cache_test.go

@@ -34,6 +34,18 @@ func TestNewContext(t *testing.T) {
 	assert.Nil(t, con)
 }
 
+func TestTest(t *testing.T) {
+	defaultCache = nil
+	_, err := Test()
+	assert.Error(t, err)
+
+	createTestCache()
+	elapsed, err := Test()
+	assert.NoError(t, err)
+	// mem cache should take from 300ns up to 1ms on modern hardware ...
+	assert.Less(t, elapsed, SlowCacheThreshold)
+}
+
 func TestGetCache(t *testing.T) {
 	createTestCache()