std.Build.Step.Run: many enhancements

This is a major refactor to `Step.Run` which adds new functionality,
primarily to the execution of Zig tests.

* All tests are run, even if a test crashes. This happens through the
  same mechanism as timeouts where the test processes is repeatedly
  respawned as needed.
* The build status output is more precise. For each unit test, it
  differentiates pass, skip, fail, crash, and timeout. Memory leaks are
  reported separately, as they do not indicate a test's "status", but
  are rather an additional property (a test with leaks may still pass!).
* The number of memory leaks is tracked and reported, both per-test and
  for a whole `Run` step.
* Reporting is made clearer when a step is failed solely due to error
  logs (`std.log.err`) where every unit test passed.

0 files changed, 0 insertions, 0 deletions