Flaky tests on iOS

[enricogior]

Some tests show random failures when executed on the iOS simulator and in some rare case on the device as well.
There are two types of failures:
F-S = flaky when run standalone.
F-A = flaky when run after other tests.
The failures when run after other tests are presumably caused by the fact that the test app doesn't fully isolate each test.

#	Test	iOS 9 sim	iOS 9 device	iOS 10 sim	iOS 10 device
109	test-dgram-udp4.js	F-A		F-A
155	test-fs-empty-readStream.js	F-A		F-A
245	test-http-exit-delay.js	F-A	F-A
382	test-net-settimeout.js			F-S
547	test-timers-unref-active.js	F-S		F-S
551	test-timers-unrefd-interval-still-fires.js			F-S