Currently, tests are scattered throughout multiple directories, making
it difficult to determine what is tested and what is not tested.
With upcoming work on interoperability between Swift and C++
(https://forums.swift.org/t/manifesto-interoperability-between-swift-and-c/33874),
we'd like to rearrange the tests and bring tests covering a single C/C++
feature next to each other.
This PR also adds tests for enabling C++ interop where possible.
This splits up the test into two halves in order to allow the test to
pass on Windows. Loosen the test to allow the enumeration to be
imported with signed and unsigned types as Windows uses signed
enumerations by default.
