Intermittent mac timeout due to missing font URLs (No URL for Core Text font!)

[jdm]

This affects lots of different tests, so I'm not adding any tests to the summary.

 ▶ TIMEOUT [expected FAIL] /css/css-fonts/font-variant-numeric-08.html
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  │ No URL for Core Text font! (thread FontCacheThread, at src/libcore/option.rs:1185)
  │ stack backtrace:
  │    0: backtrace::backtrace::trace
  │    1: backtrace::capture::Backtrace::new
  │    2: servo::main::{{closure}}
  │    3: std::panicking::rust_panic_with_hook
  │    4: std::panicking::continue_panic_fmt
  │    5: rust_begin_unwind
  │    6: core::panicking::panic_fmt
  │    7: core::option::expect_failed
  │    8: gfx::platform::macos::font_template::FontTemplateData::bytes
  │    9: gfx::font_cache_thread::FontCache::run
  │   10: std::sys_common::backtrace::__rust_begin_short_backtrace
  │   11: std::panicking::try::do_call
  │   12: __rust_maybe_catch_panic
  │   13: core::ops::function::FnOnce::call_once{{vtable.shim}}
  │   14: <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once
  │   15: std::sys::unix::thread::Thread::new::thread_start
  │   16: _pthread_body
  │   17: _pthread_start