test-cases: remove multi-threading

This effectively reverts 22690efcc2378222503cb8aaad26a6f4a539f5aa,
re-opening #11818.

This had the following problems:
 * Buggy on some targets (macOS, Windows)
 * Messy output to the terminal

These problems need to be solved before moving forward with this.
9f1f60fd43Andrew Kelley committed on 8/11/2022, 7:46:44 AMparent933436d
1 file changedLine totals unavailable