Create `tmp/run` directory if it doesn't exist in gitaly_spawn_test

What does this MR do and why?

Closes gitlab-com/gl-infra/delivery#2409 (closed)

Due to how canonical foss version of GitLab gets created, this directory sometimes does not exist in the foss version of GitLab, and causes Gitaly tests to fail.

This change modifies the Gitaly test helper so that it will check if that directory doesn't exist and create it.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

1. In a checkout of the project remove the tmp/run directory rm -rf tmp/run
2. Run the script to test Gitaly spawn ./scripts/gitaly-test-spawn
3. Confirm that tests pass