Add PInvoke/DllImportpath tests (#19270)

Add PInvoke/DllImportpath tests (#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.