Both vmalert and vmalert-tool support multiple `rule_files` and use
directory as a file, so it's ok if some files don't contain any rule
group. But vmalert-tool should warn the user if no rule group is found
in any of the `rule_files`.
Related issue:
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/7663
This makes test code more clear and reduces the number of code lines by 500.
This also simplifies debugging tests. See https://itnext.io/f-tests-as-a-replacement-for-table-driven-tests-in-go-8814a8b19e9e
While at it, consistently use t.Fatal* instead of t.Error* across tests, since t.Error*
requires more boilerplate code, which can result in additional bugs inside tests.
While t.Error* allows writing logging errors for the same, this doesn't simplify fixing
broken tests most of the time.
This is a follow-up for a9525da8a4