Post identical failures as discussion comments

Currently, all failures are reported with an error and stack trace, even if the same error and stack trace have already been reported in that issue.

This adds those new identical failures as replies to the first failure, but without including the stack trace again.

Here's a couple of examples (used the same pipeline and job URL each time, so you can ignore them):

• One discussion with a few replies: mlapierre-test/testcases-test#493 (comment 305903912)
• A new issue after slightly altering the title, and a few discussions after slightly altering the error and/or stack trace: mlapierre-test/testcases-test#495

Closes https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/438