The test that failed only when run alone

The usual failure is a test that passes alone and fails in the suite; this was the reverse, and the reverse is more interesting.

$ vendor/bin/phpunit --filter testCalculatesVat
  FAILED

$ vendor/bin/phpunit tests/Pricing
  OK

why: an earlier test in the same directory seeded a
currency configuration row that this test needed and
never created. it had been passing on somebody else's
setup for two years.

A test that depends on another test’s side effect is a hidden ordering requirement, and running the suite in a random order is what surfaces the whole class — --order-by=random with a recorded seed found six more. The fix is not making the tests independent by copying setup around; it is naming what the test actually needs and building it.