Avoid warning for system setting when start up (#23054) (#23116)

Backport #23054

Partially fix #23050

After #22294 merged, it always has a warning log like `cannot get
context cache` when starting up. This should not affect any real life
but it's annoying. This PR will fix the problem. That means when
starting up, getting the system settings will not try from the cache but
will read from the database directly.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>

models/unittest/testdb.go

@@ -113,7 +113,7 @@ func MainTest(m *testing.M, testOpts *TestOptions) {
 	if err = storage.Init(); err != nil {
 		fatalTestError("storage.Init: %v\n", err)
 	}
-	if err = system_model.Init(); err != nil {
+	if err = system_model.Init(db.DefaultContext); err != nil {
 		fatalTestError("models.Init: %v\n", err)
 	}