Test Helpers (t.Helper)

When the same assertion pattern repeats across a dozen tests, extracting it into a function is the obvious move. The problem is that a naive helper function buries the failure: t.Errorf reports the line number where it was called, which is the helper, not the test. t.Helper() fixes that by telling the test runner to skip the current function when picking a line number to blame, so failures point at the test that called the helper.