Speed testing LONG_TESTS on GitHub Actions

[terriko]

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[terriko]

Note to self: short tests take 2m, long_tests take 5m, so they're not that long.

[terriko]

2 tests at 2m each = 4m
2 tests at 5m each = 10 minutes
2 tests at 2m + 1 test at 5m = 9m

That's not entirely fair because sometimes there's also some delay before the tests run, but I think I'm going with "we're adding python3.8 tests and also they'll run the long tests" anyhow.

We're still more than doubling the time it does to run the tests, which is annoying, but I think it will help with other problems, and it will eventually be required that we run long_tests at least once for coverage anyhow.