I don’t want the top tab that says “(Failed To Load Script Object)” to appear. Also, the attributes for the imported asset are greyed out. Here are some pictures:

Note: The problem is not specific to this example but to every asset file that I use a SciptedImporter for. Also, re-installing Unity doesn’t change anything.
You have to make sure the file that contains your TestImporter class is actually named TestImporter.cs
This is the same limitation that applies to all Unity Engine objects such as ScriptableObjects or MonoBehaviours.
After you have changed the filename make sure to trigger a reimport either by increasing the number or by simply reimporting the asset.
I think you cannot actually change the variables on an imported asset as that defeats the purpose of deterministic imports. If you want to change something about the import process it should be done via variables on the importer itself which will appear at the top section where it currenctly says Script File not Found