test(stdlib): add more metaquery tests

[wolffcm]

These add tests that exercise the tagValues and tagKeys storage APIs.

This is in response to an issue found after upgrading Flux in OSS:
influxdata/influxdb#19561

And also two issues that were filed as a result of that issue:
influxdata/influxdb#19794
influxdata/influxdb#19806

The new tests added here would fail if PushDownReadTagValuesRule and PushDownReadTagKeysRule were re-enabled in OSS, so they will serve as useful regression tests for the above issues.

Fixes influxdata/influxdb#19561.

(Doesn't actually fix it, but I want that issue closed when this merges)

[codecov-io]

Codecov Report

Merging #3272 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3272      +/-   ##
==========================================
- Coverage   49.64%   49.63%   -0.01%     
==========================================
  Files         344      344              
  Lines       35855    35855              
==========================================
- Hits        17799    17797       -2     
- Misses      15582    15583       +1     
- Partials     2474     2475       +1     

Impacted Files	Coverage Δ
stdlib/universe/pivot.gen.go	9.66% <0.00%> (-0.27%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4ea803e...8b7b2cb. Read the comment docs.