Ability to run cargo tests on main thread.

[VladasZ]

Problem

While working with winit or glfw I would like to use regular cargo test to test some screens or views. The problem is that macOS requires all the GUI stuff to be initialized from main thread and these tests panic.
winit:

thread 'interface::tempo_control::test_tempo_control' panicked at /Users/vladas/.cargo/registry/src/index.crates.io-6f17d22bba15001f/winit-0.30.4/src/platform_impl/macos/event_loop.rs:214:14:
on macOS, `EventLoop` must be created on the main thread!

glfw:

*** Assertion failure in -[NSMenu _setMenuName:], NSMenu.m:777

Proposed Solution

Add the ability for cargo tests to be run on the main thread. It may look something like this:

#[test]
#[main_thread]
fn test() {}

Notes

This issue bothers many Rust developers. Here are some related issues:

gtk-rs/gtk4-rs#1235
rust-lang/cargo#3927
rust-lang/cargo#5438
https://stackoverflow.com/questions/43458194/is-there-any-way-to-tell-cargo-to-run-its-tests-on-the-main-thread

[epage]

As this is a function of the test harness, libtest, and not cargo test, I'm going to move this to appropriate repo.

[RalfJung]

This is a duplicate of #104053.