attribute: make TestHashKVs linear-time

[pellared]

Unblocks #8166

Per #8166 (comment)

Why

This updates TestHashKVsEquality to avoid the quadratic pairwise comparison that was causing slow runs and timeouts in CI.

Below are the results from my machine.

Old:

$ go test -run=TestHashKVs -count=1 
PASS
ok      go.opentelemetry.io/otel/attribute      2.063s

New:

$ go test -run=TestHashKVs -count=1 
PASS
ok      go.opentelemetry.io/otel/attribute      0.024s

Instead of collecting every generated testcase and comparing each hash against every other hash, the test now checks uniqueness as each case is generated by storing previously seen hashes in a map. Note that it also does not use the equality operator of KeyValue to determine if the hash should be equal or different.

What

• Rename the test from TestHashKVsEquality to TestHashKVs
• Replace the O(n^2) post-processing loop with an O(n) streaming uniqueness check
• Simplify failure reporting by removing the dedicated msg helper

Side note: for me it also makes the test more readable (however, this is opinionated).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.3%. Comparing base (3ee1d5a) to head (6d8095d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8204   +/-   ##
=====================================
  Coverage   82.3%   82.3%           
=====================================
  Files        310     310           
  Lines      24266   24266           
=====================================
+ Hits       19985   19986    +1     
+ Misses      3904    3903    -1     
  Partials     377     377           

see 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.