[bindings] Fix most Python binding unittests on Windows

Summary:
This fixes all but one of the test cases for Windows. TestCDB will
take more work to debug, as CompilationDatabase seems not to work correctly.

Reviewers: bkramer, wanders, jbcoe

Reviewed By: bkramer, jbcoe

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D47864

Patch written by ethanhs (Ethan)

llvm-svn: 335282