Adds the ``nightly_only_test`` and ``stable_only_test`` attribute macros
that prevent or allow certain tests to compile on nightly and stable
respectively. This is achieved through conditionally outputting the
tests TokenStream.
If CFG_RELEASE_CHANNEL is not set, it's assumed that we're running in a
nightly environment.
To mark a test as nightly only:
#[nightly_only_test]
#[test]
fn only_run_on_nightly() {
...
}
To mark a test a stable only:
#[stable_only_test]
#[test]
fn only_run_on_stable() {
...
}
