Add PInvoke/DllImportpath tests (dotnet/coreclr#19270)
* Add PInvoke/DllImportpath tests
* Refactor test to current toolset and remove Windows-only Path API searching.
* Remove explicit calling convention from managed and add correct convention to test.
* Get environment test passing on Unix.
* Update test to pass off-Windows.
* Include feature/platform defines in tests.
* Fix environment test (again).
* Remove zero-width character that's breaking CI
* Fix zero-width character I missed.
* Remove invalid parens around LD_LIBRARY_PATH
* Ensure the path added to the LD_LIBRARY_PATH exists before the test starts.
* Combine all DllImportPath CMakeLists.txt files into one file.
* Include clr.coreclr.props when also including clr.defines.targets so we get the feature defines correctly as well.
* Disable unicode test for our OSX 10.12 CI builds.
Commit migrated from https://github.com/dotnet/coreclr/commit/
9d2f9c4b4eebcac13247fa414bc8ca1c3a1d5b03