Provide validation test of git_config argument to easyblock.obtain_file()

[kelseymh]

In PR #3294, @boegel suggested adding an explicit test to verify all the git_config options that can be used in obtain_file().

easybuild-framework/easybuild/framework/easyblock.py

Line 582 in ed5ba4a

def obtain_file(self, filename, extension=False, urls=None, download_filename=None, force_download=False,

This would coordinate with the addition of git_config support for extensions, and updating the exts_list documentation (PR #617).

[kelseymh]

The git_config interface is already tested as part of filetools.py

easybuild-framework/test/framework/filetools.py

Line 2162 in ed5ba4a

def test_get_source_tarball_from_git(self):