Improve e2e spec retry
What does this MR do and why?
Fixes retry logic, improve logging and cache junit_merge
gem.
It seems that when retry run fails, script terminates early and doesn't end up merging junit reports correctly (https://gitlab.com/gitlab-org/gitlab/-/jobs/6680448681#L1591). This change should fix that.
Also moves junit_merge
gem to Gemfile so it is cached and adds extra messages for retry.
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Part of: gitlab-org/quality/quality-engineering/team-tasks#2268 (closed)