Add tests for local GCP config_dir (#629)
Add tests for local GCP config_dir (#629)

These tests help eliminate the module from the reported gaps in test
coverage when checking test coverage.
They ensure that
- config_dir is computed correctly on Windows
- config_dir is computed correctly on non-Windows platforms
- getting the config_dir is an error if LOCALAPPDATA is not set on
  Windows, if no config_dir was passed on init
- getting the config_dir is not an error if LOCALAPPDATA is not set on
  Windows, if config_dir was passed on init
globus/globus-sdk-python,globus/globus-sdk-python