integrationservertest: Add detailed reporting to Slack

Current Slack message that is reported whenever the tests pass / fail is very basic.

Implement a detailed breakdown (perhaps in the form of a table) that shows the tests that were run as well as their status, so that we can easily tell which tests failed, which tests were run (were they expected?), whether we accidentally skipped some tests etc.

Example:

| Test | Status |
|-----|--------|
| Upgrade 8.15 -> 8.16 -> 8.17 | ✅ |
| Standalone-to-managed 7.17 -> 8.19 -> 9.1 | ❌ |
| Some other test | SKIP |