The workhorse integration tests for Gitaly need to have a Gitaly process
running. It uses scripts/setup-test-env to build Gitaly, but it used a
home-grown solution to start the process.

This change switches to the "standard way" of starting Gitaly in test
for workhorse, so it has the same environment variables when set up and
started. This has an added benefit as now the Gitaly socket is tested
before the workhorse tests start.