Labs
Three things a mutation board alone will not teach you: how to write a rule instead of a case, how to keep a test double honest, and how to find the variance in a suite that only mostly passes.
Property drills
Write an invariant instead of examples. A seeded generator fuzzes it and shrinks any counterexample to the smallest input that still fails.
Doubles lab
Stub, mock, fake, spy — and the failure mode all four share. A double is a claim about the real thing, and nothing checks it unless you do.
Flake lab
A deterministic scheduler injects ordering and randomness variance. Make the suite deterministic; the engine runs it under a hundred seeds and reports the rate.