Add array-testing assertions

[matatk]

We discussed and @MarkWithall suggested these offline; they ought to be noted here too :-)...

• ASSERT_ARRAY_EQUAL(actual, expected, length, message)
• ASSERT_ARRAY_LENGTH(array, expected_length, message)

I'm not able to 100% guarantee that I'd have the cycles or be able to do it correctly, but I might be able to have a go at this, so feel free to assign to me if you would like a PR.

Currently @tomfinnie and I are testing arrays and will be using a for loop, which I gather will work in the sense that we can have more than one ASSERT per TEST, but I gather that it would only break on the first ASSERT as-is. Specifically the thing we're trying to do is assert that one (and only one) element in the array has been set to NULL -- that's actually not quite covered by the above, I think (we don't know/care what the other values are), so this specific behaviour looks like it'll be extra to the above-listed assertions.

[MarkWithall]

Just a thought: I don't believe one can check the length of an array in C, as an array is just a pointer to the starting memory location and a type; with no other information.

[matatk]

Ah, yes, of course. I think that ASSERT_ARRAY_EQUAL would still be do-able, with great care. Maybe even ASSERT_ARRAY_LENGTH one, with even greater care (though perhaps it's not possible practicably), but perhaps it's worth leaving these to the application for now (we were able to use a for loop, as described).