Add some tests to clarify the "must-change-password" behavior (#30693)

Follow  #30472:

When a user is created by command line `./gitea admin user create`:

Old behavior before #30472: the first user (admin or non-admin) doesn't
need to change password.

Revert to the old behavior before #30472

modules/log/logger_global.go

@@ -57,11 +57,13 @@ func Critical(format string, v ...any) {
 	Log(1, ERROR, format, v...)
 }
 
+var OsExiter = os.Exit
+
 // Fatal records fatal log and exit process
 func Fatal(format string, v ...any) {
 	Log(1, FATAL, format, v...)
 	GetManager().Close()
-	os.Exit(1)
+	OsExiter(1)
 }
 
 func GetLogger(name string) Logger {