MaxListenersExceededWarning for beforeExit event

[novemberborn]

Sequence listens for the beforeExit event. This is done for every test in the worker. If there are more than 10 tests this causes a MaxListenersExceededWarning to be reported.

See

ava/lib/sequence.js

Line 22 in 8f80ed1

process.on('beforeExit', onBeforeExit);

.

We need to manage beforeExit subscriptions so that we only listen on process once, but still allow multiple callbacks due to concurrent Sequence runs.