Add PInvoke/ExactSpelling tests (#19303)

Add PInvoke/ExactSpelling tests (#19303)

* Add PInvoke/ExactSpelling tests

Refactor tests to fit with the rest of the Interop tests.

Fix up test to cleanly run.

Change CMakeLists.txt to match the rest of the tests.

Include Interop.cmake in CMakeLists.txt

Remove Service.

* On x86 enable stdcall mangling irrespective of ExactSpelling and account for the charset suffix when ExactSpelling = false.

Change variable name.

Clean up the FindEntryPoint. The logic flow now matches CoreRT + CoreCLR specific features (ordinals and stdcall mangling).

PR Feedback.

Fix format specifier.

Add back probing null check.

Fix offset calculation for stdcall mangling.

Probe the stdcall-mangled versions of the original entry-point names when ExactSpelling isn't set.

Cleanup.