Add WillCompleteOnCancel expectation

[cleroux]

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Closes #299

I nerd-sniped myself after writing up the issue and started looking into the code. Turned out to be pretty straight-forward. So here's a PR! I hope you think it's useful. If you decide to merge this I would certainly pull this update and use this feature to write some new tests.

The goal of .WillCompleteOnCancel() is to be able to test a pretty rare edge case in Postgres (and maybe other databases?) where the Go context is cancelled but due to timing, may not succeed in actually cancelling the query. In this case the query completes without error, and functions like QueryContext() et al. do not return the "context canceled" error we normally expect.

This change is fully backward-compatible. There is no change to the default behavior which is to return ErrCancelled when the context is cancelled.