Node sets a test's concurrency to Infinity for `true` whenever the test
has a parent, and to os.availableParallelism() - 1 only for the root
test. Every test and suite a program declares has a parent, so `true`
now runs all of its subtests at once instead of capping them at the CPU
count minus one.
The concurrency fixture adds a suite of 64 concurrent tests that checks
all of them run at once.