Fix FileMatcher_tests to not append spurious trailing slash
Fix FileMatcher_tests to not append spurious trailing slash

Led to certain bugs not being found, like:
https://github.com/Microsoft/msbuild/issues/917

AndyGerlicher/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,nikson/msbuild,chipitsine/msbuild,AndyGerlicher/msbuild,mono/msbuild,chipitsine/msbuild,Microsoft/msbuild,cdmihai/msbuild,cdmihai/msbuild,jeffkl/msbuild,nikson/msbuild,Microsoft/msbuild,AndyGerlicher/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,Microsoft/msbuild,rainersigwald/msbuild,mono/msbuild,cdmihai/msbuild,jeffkl/msbuild,mono/msbuild,jeffkl/msbuild,nikson/msbuild,nikson/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,rainersigwald/msbuild,rainersigwald/msbuild,mono/msbuild,sean-gilliam/msbuild