ed4d38fc

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Mutation: order covering tests cheapest-first in the mutation child

When a mutation child runs the tests that cover a mutation, it runs them
synchronously under fail-fast, so the first covering test that fails kills the
mutant.  Ordering the covering tests cheapest-first, using the per-test baseline
the coverage phase already measures (and previously discarded into a single
timeout), reaches a killing test sooner and cuts the mutation phase's wall-clock,
since killed mutants are the overwhelming majority of a run.  On two sample
projects the mutation phase ran ~6% and ~24% faster, with identical verdicts.

- reorderTestForestByTiming: a pure, tree-aware, stable reorder.  It sorts
  siblings by ascending baseline (missing = first) and orders each subtree by the
  minimum cost of any leaf it contains, so a shared-setup group reorders as a
  unit and setup is never split.  It mirrors randomiseTestForest: inside a
  DoNotRandomiseExecutionOrder scope it leaves the subtree untouched at every
  depth.  So it is a drop-in alternative to execution-order randomisation, legal
  under exactly the same conditions.  Only sibling order changes, so which tests
  run (and every verdict) is unaffected.
- reorderForMutationChild: the pure gate the child uses -- reorder only when the
  suite randomises execution order and a baseline is available.  Keeping it out
  of the child's IO makes the gate unit-testable.
- The coverage phase persists the merged per-test baselines as baseline.json next
  to the manifest; the run and diff paths union them into the augmented dir the
  children read.
- The child reads that baseline and reorders the filtered forest after filtering,
  so the coverage-order TestIds the filter needs are untouched.

Tests: ReorderSpec covers ascending order, stable ties + missing-as-first, that a
sequential (parallelism) subtree is reordered but a doNotRandomiseExecutionOrder
one is not (incl. no descent into a nested randomise block), that a beforeAll_
group reorders as a unit without splitting setup, that the test-id set is
unchanged, and the gate (on/off/no-baseline).

Timeline