Fix CI pipeline test reports

As part of !4254 (merged) we started running tests as an unprivileged user, and so had to move the test and coverage reports to /tmp.

It turns out artifacts cannot be collected from /tmp as these temporary files appear to be removed before gitlab CI collects them. This results in:

Uploading artifacts...
WARNING: /tmp/go-tests-report.xml: no matching files
ERROR: No files to upload

To fix this we first create an unprivileged reports/coverage directory as part of preparing for tests.

Test reports can be seen https://gitlab.com/gitlab-org/gitaly/-/pipelines/548365149/test_report

Coverage is a bit harder - we'd need some go code...

I really wish we could tell gitlab-ci to explode when artifacts fail so that we notice.