Cedar silently accepts misuse of the test structure DSL

[briancroom]

The following code runs, with the incorrectly-placed nested contexts being silently ignored:

it(@"has an outer test", ^{            
    context(@"when a context is nested inside of an 'it'", ^{
        it(@"blah", ^{
            1 should equal(2); // This test is not executed
        });
    });
});

Cedar should probably just throw an exception if context, describe, it etc. are called while a test is being executed.