test: simplify test helpers

[mccutchen]

Simplify our internal assert package, now lightly adapted from this blog post. Primary benefits:

• Unified assert.Equal() and assert.DeepEqual() implementation
• Better assert.Error() implementation, which handles error types using errors.As()
• Plus, just deleting a bunch of unused test helper code

[github-actions]

🔥 Run benchmarks comparing d364a41 against main:

gh workflow run bench.yaml -f pr_number=70

Note: this comment will update with each new commit.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.11%. Comparing base (d008872) to head (d364a41).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #70   +/-   ##
=======================================
  Coverage   98.11%   98.11%           
=======================================
  Files           2        2           
  Lines         425      425           
=======================================
  Hits          417      417           
  Misses          4        4           
  Partials        4        4           

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