Improve `autoTestDiscoverOnSavePattern` to be more robust

[karthiknadig]

You might look at our files.exclude / search.exclude settings for a different structure. It's an object with glob keys which lets you layer the setting, turning off or on individual patterns at different settings scopes. Like I might want to set some glob for autoTestDiscoverOnSavePattern in my user settings, and another one in my workspace settings, and have both applied?

Originally posted by @roblourens in #24817