You can easily test your masterlist changes before syncing them to the online repository, using the method detailed below, which makes LOOT use a local masterlist of your choosing rather than the online one.

1. Run LOOT, open the File menu and select Settings....
2. Select the game you want to test and, in the Masterlist Source field, enter the path to your local masterlist. E.g., if you have the Skyrim masterlist in a repository at C:\foo\bar\skyrim\masterlist.yaml, then put that in the field.
3. Save your settings, and the settings dialog will automatically close. LOOT will now update using your local masterlist instead of the online one.

Note that LOOT won't keep your local masterlist synced with the online one, so you'll have to do that yourself.