[windows] Add folder to path while testing.

[drodriguez]

Some DLL are not installed, since they are only used for testing, but
they need to be in the PATH to allow some test binaries to execute
properly.

The test happened in CI because _InternalSwiftSyntaxParser.dll is not installed, and sits in %build_root%\swift\bin.

lit.py: C:\jenkins\workspace\oss-swift-windows-x86_64\llvm\utils\lit\lit\formats\googletest.py:43: warning: unable to discover google-tests in 'S:\\swift\\unittests\\SyntaxParser\\.\\SwiftSyntaxParserTests.exe': Command '['S:\\swift\\unittests\\SyntaxParser\\.\\SwiftSyntaxParserTests.exe', '--gtest_list_tests']' returned non-zero exit status -1073741515. Process output:

[drodriguez]

The Error 87 will probably still happen. The error from 27823 should still happen, as well as 27843 (intentionally not linking those two to avoid problems with the CI bot).

@swift-ci please test Windows platform

[drodriguez]

@swift-ci please smoke test