Implemented: Make Git tests run with external, local repository.

It is not possible to locate a Git repository inside another Git
repository. Therefore, the tests now extract a ZIP file with the test
repository into the system temp dir and use that one. The original
testing repository was re-added in a ZIP file.
Refactored: Management of ZIPed repositories into more generic test base.

Preparation to handle all repositories the same way.
Fixed: Make test error on invalid BZR install.

The test failed before, if BZR "whoami" was not set. This fix makes the
test run into a meaningful error instead.
Implemented: Re-added fixtures for all supported systems and migrated tests.
Merge pull request #2 from tobyS/test-fixtures

Test fixtures