Gitaly comparison tests: made non-blocking with a startup timeout

If, for some reason, Gitaly failed to start, the tests run was
blocking in a nasty way, requiring SIGTERM to kill the process.

One common reason is to forget to activate rbenv. Another one
seems to be a HDK Gitaly service to be running on the same
system, that we may now be able to investigate comfortably.

This solves the problem with a timeout, defaulting to 5 seconds,
and further tweakable with the `GITALY_STARTUP_TIMEOUT` environment

Note: 2 seconds would be too short already on my workstation.
2 jobs for !37 with topic/default/gitaly-startup-timeout in 31 seconds
latest detached
Status Job ID Name Coverage
failed #170010
allowed to fail


passed #170009


Name Stage Failure
tests-hg-default Compat
    writing top-level names to pip-egg-info/mercurial.egg-info/top_level.txt
writing manifest file 'pip-egg-info/mercurial.egg-info/SOURCES.txt'
Python headers are required to build Mercurial but weren't found in /usr/include/python3.7m/Python.h

Command "python egg_info" failed with error code 1 in /tmp/pip-install-6ibzg4y2/Mercurial/
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1