[TESTS] enable AddFixtures in unit tests

Use setting.AppWorkPath instead of filepath.Dir(setting.AppPath). It
is the common denominator between:

* models/unittest/testdb.go:MainTest
* tests/test_utils.go:InitTest

which makes it usable in unit tests as well as integration tests.

tests/test_utils.go

@@ -271,8 +271,8 @@ func Printf(format string, args ...any) {
 func AddFixtures(dirs ...string) func() {
 	return unittest.OverrideFixtures(
 		unittest.FixturesOptions{
-			Dir:  filepath.Join(filepath.Dir(setting.AppPath), "models/fixtures/"),
-			Base: filepath.Dir(setting.AppPath),
+			Dir:  filepath.Join(setting.AppWorkPath, "models/fixtures/"),
+			Base: setting.AppWorkPath,
 			Dirs: dirs,
 		},
 	)