intermittent tests (mostly?) fixed

[Robert O'Callahan]

I've done a lot of work over the past several weeks to track down various intermittent test failures and fix them. (The main issues were around task shutdown in unusual situations, especially involving SIGKILL being delivered at random times. There was also an issue with all tests involving detaching that would occasionally cause the detached task to be in a group-stop, hanging the test.) Right now we seem to be in a pretty good state, e.g. the last 7 commits (up to and including 495b1b27aaa2a419bb371d84f1838adfc2b80896) are all green.

From now on we should try to keep CI green. If anyone sees red on CI we should figure out what the problem is and fix it.

Rob

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