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.
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