Intermittent failure in `partial/mixed` test

[garyb]

This pops up once every couple of hundred runs of the test case

D:\Home\GitHub\updates-slamdata\purescript-aff\output\Effect.Aff\foreign.js:513
                throw util.fromLeft(step);
                ^

Error: Assertion failure parallel/mixed

[natefaubion]

Since the test relies on timing effects, and the failures are intermittent, it's probably a bad test.