Macro musli_tests::if_supported

macro_rules! if_supported {
    (musli_zerocopy, lg, $block:block) => { ... };
    (musli_zerocopy, allocated, $block:block) => { ... };
    (musli_zerocopy, medium_enum, $block:block) => { ... };
    ($framework:ident, $test:ident, $block:block) => { ... };
}

Only expand $block if the given test is supported by this framework.