[bindings] Fix most Python binding unittests on Windows
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
Differential Revision: https://reviews.llvm.org/D47864
Patch written by ethanhs (Ethan)