We have recently migrated Gitaly to use a new runtime directory instead
of its old internal socket directory. One outcome of this migration is
that internal Unix socket paths as created by test Gitaly instances are
now longer than they had been before because they're created inside of a
subdirectory. Due to limitations on the maximum socket path length, this
is causing setup failures on some developer's machines depending on the
location of the GDK.

Let's improve this situation and give developers a little bit more
headroom by migrating Gitaly's runtime directory to `tmp/run` instead of
nesting it into `tmp/tests/gitaly`. This runtime directory can be shared
across all Gitalies because Gitaly creates a directory `gitaly-$PID` in
the runtime directory.

This change buys us another 9 characters of headroom towards the 104
character limit for socket paths.