Tests create global config stub if required

Instead of relying on the global configuration and skipping the test if
Git is not installed, the following tests create a configuration file
containing user.name and user.email using
BaseFixture.CreateConfigurationWithDummyUser(Signature):

* CommitFixture.CanCommitWithSignatureFromConfig()
* ConfigurationFixture.CanEnumerateGlobalConfig()

Fixes #561