Better tests

[georgefst]

The evtest executable is useful as a quick way to check that we haven't totally broken everything, but we really ought to have some more extensive tests.

It would be nice to set up continuous integration, especially since hackage-ci is useless for us. Done.

[georgefst]

dfbfd20 is a useful start, but coverage is still low.

[georgefst]

We should possibly move the tests, so that we can also test the streamly stuff in one suite.

(If we do this, we may as well use streamly for concurrency in the existing tests, instead of manually using forkIO and MVars.)

[georgefst]

hackage-ci is useless for us

Incidentally, this is no longer the case, since haskell-infra/hackage-doc-builder-config@a65efdd. Which, given the timing, must have been from @gbaz seeing this Reddit thread.