Write tests that assert modreq breaking support for native csc/vbc

[OmarTawfik]

We're using modreqs to prevent other/older compilers from consuming/extending ref-readonly symbols in a non-safe way.
There is already some tests being added on VB side to assert that behavior, but we need to write [WindowsOnly] tests that call the csc/vbc on the machine (the native compilers) and assert they exhibit the right behavior as well.

See #17287 for more details

[jcouv]

I'm not sure what is the value of writing such test automation. Shouldn't it be good enough to verify once (possibly against different versions of the compiler)? Those shipped compilers are not expected to change.

[OmarTawfik]

@jcouv manual verification was done. But I want to have this sort of automation in case a regression happened in the way the current version writes out modreqs. This is not a high-pri though.

[tannergooding]

This should probably be an outer loop test.

It should also probably be added to the list of manual checks done prior to release.