### Describe Your Changes
These are the integration tests that confirm that instant queries may
return stale NaNs when the query contains a rollup function.
The bug was reported at #5806. There is also a fix: #7275. The tests in
this PR will be used co confirm that the fix works.
Some test refactoring has been done along the way. Sorry, couldn't
resist.
### Checklist
The following checks are **mandatory**:
- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).
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Signed-off-by: Artem Fetishev <rtm@victoriametrics.com>
