Using a Promise in BackgroundAction was not doing anything since the
change to use a weak reference to the event loop, so let's just drop
that.
The thread will now always move itself (and therefore its callbacks)
over to the originating thread before completing, regardless of the
presence of callbacks. This ensures that ref counting remains on the
main thread.
In addition, BackgroundAction's completion callback can no longer
return errors. This functionality wasn't actually used anywhere, it was
a holdover from the behavior of Core::Promise.
The thread pool test is currently flakey and takes over 2 minutes to run
on CI. It also currently has no users now that RequestServer uses curl,
so let's just remove it for now. If we need it in the future, we can
revive it from git history.
The threading tests currently wait for a very small amount of time for
the expected test condition to be reached, e.g. 20ms. This changes the
tests to *check* the condition every 20ms, but allow the test to run for
up to 2s until the condition is reached. This should hopefully resolve
the failures seen on CI.
This also renames one of the tests to match what it actually does. The
test itself was changed in commit 5b335e7, but the name was not updated
to reflect that change.
Note that in some cases (in particular SQL::Result and PDFErrorOr),
there is no Formatter defined for the error type, hence TRY_OR_FAIL
cannot work as-is. Furthermore, this commit leaves untouched the places
where MUST could be replaced by TRY_OR_FAIL.
Inspired by:
https://github.com/SerenityOS/serenity/pull/18710#discussion_r1186892445
Using a file(GLOB) to find all the test files in a directory is an easy
hack to get things started, but has some drawbacks. Namely, if you add
a test, it won't be found again without re-running CMake. `ninja` seems
to do this automatically, but it would be nice to one day stop seeing it
rechecking our globbed directories.
