fix massive race in testBootstrapsCallInitializersOnCorrectEventLoop (#504)
fix massive race in testBootstrapsCallInitializersOnCorrectEventLoop (#504)

Motivation:

testBootstrapsCallInitializersOnCorrectEventLoop had a massive race
condition, we'd never wait for childChannelDone and serverChannelDone to
actually be fulfilled which could lead to leaked promises. No idea why
this didn't fail more often.

Modifications:

Wait for futures to be fulfilled to make sure we don't leak the
promises.

Result:

tests more stable.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio