Conditionally compile tests based on CFG_RELEASE_CHANNEL env var

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() {
        ...
    }

0 files changed, 0 insertions, 0 deletions