tests(metrics_extraction): Various test improvements

[armenzg]

The following changes:

• Drop APDEX_THRESHOLD_DEFAULT and use DEFAULT_PROJECT_THRESHOLD
• Clean up some tests for improved readability
• Split up _on_demand_query_check for single responsibility principle

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a1ae836) 81.40% compared to head (65c7e23) 81.40%. Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #65012      +/-   ##
==========================================
- Coverage   81.40%   81.40%   -0.01%     
==========================================
  Files        5251     5251              
  Lines      232145   232145              
  Branches    45581    45581              
==========================================
- Hits       188987   188983       -4     
  Misses      37276    37276              
- Partials     5882     5886       +4     

Files	Coverage Δ
src/sentry/snuba/metrics/extraction.py	91.21% <100.00%> (ø)

... and 5 files with indirect coverage changes

[armenzg]

nit: "Various changes" in commit message is a bit ambiguous, is there an overall theme to the refactors? (or should these be separate PRs?)

Mostly making the following PRs easier to read. I'm not satisfied with the commit message either.