tst_QTimer::basic_chrono() is flaky on macOS 10.13

agent:2019/01/19 11:21:11 build.go:192: FAIL!  : tst_QTimer::basic_chrono() 'qAbs(rt.count() - 150) < 50' returned FALSE. (89)
agent:2019/01/19 11:21:11 build.go:192:    Loc: [../../../corelib/kernel/qtimer/tst_qtimer.cpp(231)]