Parallel run of the tests

As the test setup doesn't rely on the global config.Config variable anymore we are allowed to mark tests to run in parallel.
It speeds up execution, so we would wait less for the pipeline to complete.

The execution time difference recorded on my machine using command

time TEST_OPTIONS="-race -count=1" TEST_PACKAGES=./internal/gitaly/service/<PACKAGE> make test-go > test.log

package	old	new
operations	1m41.783s	1m7.152s
commit	1m42.464s	1m22.096s
remote	0m45.134s	0m41.832s
repository	1m55.610s	1m2.816s