Clean up tests under Interop/PInvoke/BestFitMapping

[elinor-fung]

• Collapse a bunch of different test projects targeting BestFitMapping
• Pull out shared code for test data and validation
• Add test cases specifically for unmappable characters (they were previously grouped with characters that did have a best-fit mapping and were commented out for some cases)

Aside from making my eyes bleed less when I look inside that folder, this reduces the time it takes to run the tests, time to upload results, and size of the uploaded test artifact for each PR:

	Before	After
Number of projects in Interop/PInvoke/BestFitMapping	48	5
Interop.PInvoke.XUnitWrapper test run (windows x64 checked)	~23s	~12s
Uploading files for Interop/PInvoke/BestFitMapping results	~14s	~1s
CoreCLRManagedTestArtifacts_AnyOS_AnyCPU_checked.zip size	47.8 MB	46.5 MB

@AaronRobinsonMSFT @jkoritzinsky - sorry, the diff is probably not very helpful... All the p/invoke definitions were direct copies/moved files. The existing methods for validation were nearly all the same with assorted discrepancies where some things were or were not checked, so I pulled that all out into a shared helper so that they would avoid said discrepancies without changing how/what they were validating.

[elinor-fung]

cc @trylek - this change seems like a manual version of the test grouping you and @jkoritzinsky are working on and I think it further reinforces how much value that work would bring.