Change opt-level from 3 to 2 for the test profile

[daira]

See #1606 (comment) for rationale.

[daira]

There is an improvement to build+test latency on CI, but it's small:

• "Test on Linux" took 5m 53s with this PR, vs 6m 2s before (a 2.5%, or 9 second improvement)
• "Test Orchard on Linux" took 7m 23s with this PR, vs 7m 39s before (a 3.5%, or 16 second improvement)

The disadvantage is that testing would not see miscompilations that happen at opt-level 3 but not opt-level 2. On reflection, I think this is a significant downside. Testing at the same optimization level as the code we release (even without LTO, which admittedly could be a source of miscompilations) seems like a good thing to maintain. So I'm inclined to close this without merging.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.30%. Comparing base (c6db09b) to head (2405364).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1607   +/-   ##
=======================================
  Coverage   56.30%   56.30%           
=======================================
  Files         148      148           
  Lines       19062    19062           
=======================================
  Hits        10732    10732           
  Misses       8330     8330           

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