feat(seer grouping): Add tests for handling of multiple Seer results (#88621)

This adds tests covering the handling of multiple Seer results to the `get_seer_similar_issues` test suite, so that we can have confidence that it works before increasing the number of matches we request from Seer. The main things these tests are trying to show are a) that the right result is picked, b) that `_should_use_seer_match_for_grouping` isn't called more times than needed, and c) that the right metrics are logged, and that those metrics are logged with the expected tags.

Author: lobsterkatie