Adding more branch coverage in client.py.

- Rename `_RequireCryptoOrDie()` to `_require_crypto_or_die()`
- Testing un-traversed GAE and GCE branches in
  `GoogleCredentials.get_application_default()`
- Missing env. var. branch in `_get_environment_variable_file`
- Missing APPDATA branch (on Windows) for `_get_well_known_file`
- Renaming some of the long ADC test names
- Adding test for `GoogleCredentials.get_application_default()`
  that actually uses the well-known file
- Branch in `GoogleCredentials.from_stream()` when filename is
  False-y
- Testing `_get_application_default_credential_GAE` and
  `_get_application_default_credential_GCE` (these just wrap
  circular imports at run time)
- Testing `_require_crypto_or_die`