spec: Shorten path of Gitaly runtime directory to mitigate socket issues

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.

Part of gitlab-development-kit#1480 (closed).