Ruler needs more comprehensive test coverage

[csmarchbanks]

Right now it is far too easy to accidentally cause ruler related issues. Add some tests that actually evaluate rules and make sure rules/alerts are evaluated correctly.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 30 days. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[jtlisi]

#1999 and #2269 added significant test coverage to the ruler.