Test annotations

[griwes]

We need to support test annotations; the initial form is supposed to work just as tags, but later

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Bikeshedding syntax proposal:

MAYFLY_ADD_TESTCASE("foo [basic] [!all]", []()
{
    MAYFLY_REQUIRE(false);
});

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Required futures:

• ./test-binary implicitly runs everything in [all]; require an explicit annotation of [!all] to exclude from execution by default
• ./test-binary -a basic runs all tests in [basic]
• ./test-binary -a '!basic' runs all tests but those in [basic]
• ./test-binary -a basic -a parallel runs all tests in [basic] and all tests in [parallel]
• ./test-binary -a 'basic+parallel' runs all tests that are tagged with both [basic] and [parallel]