Define VSToolsPath property properly

[Forgind]

Fixes #6607

Context

MSBuild theoretically defines the VSToolsPath property in its app.config/app.amd.config files, but it doesn't actually fire in practice. (You can tell since two of the properties next to it were pulled out and properly defined elsewhere, and that didn't cause "duplicate definition" errors.)

Changes Made

Move both the VSToolsPath and the MSBuildExtensionsPath64 out of the condition and give them proper values. I did have to hardcode 17.0, but that is done in several other places, so I think it appropriate.

Testing

I tried the provided repro before and after this change. This made it not happen anymore.

Also added a unit test.

[rainersigwald]

I don't think it's acceptable to remove the fallback behavior like you're doing here. Can you describe what you discovered about the problem and why you think this is the fix?

[Forgind]

Two things:
It wasn't defined in the first place, so there was no "fallback." {null} never matches where the file was.
And then the other place it looked for VSToolsPath didn't exist with the way I was testing. I can try out other scenarios to see if it's sometimes there or just add it back, since it doesn't really hurt anything. The only actually necessary change to break the repro was adding line 160/190 to app.config/app.amd.config.