fix: revert internal tests to internal package

[mpyw]

Summary

• Revert internal tests from external (foo_test) back to internal (foo) package
• Delete export_test.go files that were exporting unexported functions
• Keep t.Parallel() calls that were added

Testing unexported functions should use internal tests, not external tests with export hacks.

Test plan

• go test ./... passes locally
• CI passes

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.25%. Comparing base (66b7333) to head (d9d415f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #26   +/-   ##
=======================================
  Coverage   84.25%   84.25%           
=======================================
  Files          24       24           
  Lines        1842     1842           
=======================================
  Hits         1552     1552           
  Misses        173      173           
  Partials      117      117           

Flag	Coverage Δ
unittests	84.25% <ø> (ø)

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