ios failures when validating against xcode 9.4

[gnzlbg]

https://travis-ci.org/fitzgen/mach/jobs/415126150

It appears that the test harness is not properly handling the device output (no output is provided).

Packaging simulator app
Looking for iOS simulator
Creating iOS simulator
	running: "xcrun" "simctl" "create" "rust_ios" "com.apple.CoreSimulator.SimDeviceType.iPhone-SE" "com.apple.CoreSimulator.SimRuntime.iOS-10-2"
DD6E21B2-0C34-47D1-A42E-A2A07CEE476E
Starting iOS simulator
	running: "xcrun" "simctl" "erase" "rust_ios"
	running: "xcrun" "simctl" "boot" "rust_ios"
Installing app to simulator
	running: "xcrun" "simctl" "install" "booted" "ios_simulator_app/"
Running app
stdout --
com.rust.unittests: 2480
stderr --
Shutting down simulator
	running: "xcrun" "simctl" "shutdown" "rust_ios"
thread 'main' panicked at 'tests didn't pass', ./ci/deploy_and_run_on_ios_simulator.rs:146:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   6: deploy_and_run_on_ios_simulator::run_app_on_simulator
   7: deploy_and_run_on_ios_simulator::main
   8: std::rt::lang_start::{{closure}}
   9: std::panicking::try::do_call
  10: __rust_maybe_catch_panic
  11: std::rt::lang_start_internal
  12: std::rt::lang_start
  13: main
error: test failed, to rerun pass '--test main'