[Mac] Write libMonoPosixHelper validation tests

[chamons]

In mono/mono#7516 (comment) we broke copying in libMonoPosixHelper and yet only code stripping tests complained.

We should write some sanity "the bundle is what we expect" tests that include libMonoPosixHelper.

[spouliot]

Maybe running it's unit tests (for XM only) would do the job and offer more useful results ?

[chamons]

So i'm not sure we have any tests that describe the expected bundle structure. We just spin something up, call run on it, and if it spits out the guid we expect we call it solid.

I think we should write a test that describes expected files. Stuff like "There shall be an executable at location X, a Info.plist at Y, these libs at Z" and so on. Yeah, most of the time if you don't have those it doesn't launch, but in this case we got "lucky" that the stripping tests cared about libMonoPosixHelper else it'd be a runtime crash later.

[chamons]

Existing tests in CodeStrippingTests should also be improved to test Full (like the new Binding tests do).

[spouliot]

@chamons many of the msbuild tests do stuff like that, i.e. check the output for expected (or unexpected) files