Fix flaky teardown spec

What does this MR do and why?

Spec was dependant on directory being created. This was reproducible with rspec --seed 36645

This MR changes the test to stub the file open call so it isn't dependant on a file being created. The test now checks that file open is called against the correct log file, rather checking docker#exec is sending the right sql string. (docker#exec is tested https://gitlab.com/gitlab-org/gitlab-qa/-/blob/master/spec/gitlab/qa/docker/engine_spec.rb?ref_type=heads#L10-10 already)

How to set up and validate locally

rspec --seed 36645

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.