Refactored SampleGen test tooling

[Arlodotexe]

Until recently, tests only checked the diagnostic output of a source generator, not the actual generated code. This changed with CommunityToolkit/Labs-Windows#336 when we added the PaneOption_GeneratesTitleProperty test.

• This test also needed to check the sample registry, which is generated in the app head instead of the sample project.
• This resulted in a copy-pasted method that worked well for that test, but not for the new tests.
• We needed to properly recreate the Head-Sample project reference setup we have in our solution. These changes now enable that.

This PR contains additional improvements extracted from #44.

[michael-hawker]

Small request to update to new string literal. Otherwise, seems alright to me so far. @Arlodotexe did you test updating the main repo to this branch and seeing that it builds as expected there with these changes?

Wouldn't mind a quick pass from @Sergio0694 as well as our resident generator expert... 😉

[Sergio0694]

Had a look at the generator, changes seem fine to me under the assumption that the generator was already non-incremental for most of its pipeline and generally incredibly expensive, but my understanding is this intended 😆

[michael-hawker]

@Arlodotexe LGTM, but looks like conflicts with the bast branch now to resolve?

[Arlodotexe]

@Arlodotexe LGTM, but looks like conflicts with the bast branch now to resolve?

Had a conflict on pull at one point, so this branch contains a merge commit. Rebase has conflicts, but Squash or Merge will work fine. I'd like to be able to revert 102f46a in the future to continue that work, so I'll use a normal merge here.