docker: Remove shell based tests and add extra checks to git loader test
Shell based tests for the docker environment are now redundant with
pytest-testinfra
based ones so it is time to remove them.
I took care to implement in Python the extra shell checks testing if all git objects have been loaded into the archive, so we do not lose some coverage.
I also replaced the use of the git command by the dulwich module in test implementation.
See commit log for more details.
Migrated from D8222 (view on Phabricator)