chore: drop reflection from test suite

[mccutchen]

This should be a no-op change, but eliminates an element of the test suite that has always bothered me. There are probably further refactorings of this section of the tests to come, but don't want this diff to get any hairier than it already is.

Diff best viewed with whitespace-only changes ignored, this is mostly re-indentation:
https://github.com/mccutchen/go-httpbin/pull/227/changes?w=1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.12%. Comparing base (4a32f90) to head (e9b78c8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #227   +/-   ##
=======================================
  Coverage   95.12%   95.12%           
=======================================
  Files          10       10           
  Lines        1827     1827           
=======================================
  Hits         1738     1738           
  Misses         52       52           
  Partials       37       37           

🚀 New features to boost your workflow:

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