Use global execution context for all tests

[aeons]

Update the rest of the tests to use the global execution context instead of a single threaded executor.

[zainab-ali]

I've actually managed to reproduce this one locally (yay!).

According to the spec, calls to request after cancel should be noops. It also implies that they should be run synchronously. Unfortunately, the code for enqueueing is run asynchronously, such that the order of request / cancel operations isn't guaranteed.

This was broken before and has been lurking for a while 😞

One way of fixing this is to run enqueue synchronously with unsafeRunSync, which would mean using IO as a concrete instead of F. I'd rather not do that, and there's probably a better solution.

I'm going to merge this PR in in any case - it isn't the cause of the bug, it just surfaces it.

[aeons]

I actually thought that @rossabaker fixed this in f57c9be.

[rossabaker]

I fixed that one and unfixed it and thought it was still fixed. I think @zainab-ali is right. I'll look for my fix and comment on other ticket.

[zainab-ali]

This has been fixed in v0.2.3