test_refresh_credentials(): use UTC instead of local time.

In test_refresh_credentials(), datetime.now() is used to set the
_credential_expiry_time field of a provider. This value is later compared to
the result of datetime.utcnow() in _credentials_need_refresh(). This comparison
is meaningless. Use datetime.utcnow() in test_refresh_credentials() to fix
this.
Merge remote-tracking branch 'CyrilRoelandteNovance/fix_test_refresh_credentials' into develop
Fixes #2020.