Add macros for enhanced output in Swift Testing

[ZevEisenberg]

I didn't see a discussion or ticket for this already, which surprised me. I just found myself suggesting this change in a code review:

-expectNoDifference(tables.contains("someTableName"), true)
+#expect(tables.contains("someTableName"))

My justification was that, for simple values, the advanced diff output of expectNoDifference is not as meaningful (although still nice), and the tradeoff is that the #expect macro will include the contents of the failing expression (tables.contains("someTableName")) in the failure message in the Xcode console, sidebar, CI, etc.

But it's a drag to have to think about which one to use. I'd rather just use expectNoDifference and get the best of both worlds.

What about something like #expectNoDifference(foo, bar) or maybe #customDumpExpect(foo == bar) and #customDumpExpect(foo != bar)? The naming is up for debate, but the idea is that it means you could always use it instead of #expect without having to think about which is which. Maybe it could even support all the other operators like > from #expect, just as a passthrough, or even wrap #expect for non-== cases, and then a project could use a linter to enforce that it always uses the one from this library.

[stephencelis]

We've wondered about this a little but couldn't really figure out if it would work or not, and given the cost of macros, we decided to spare this library unless we could make a worthwhile case.

If you explore the problem a bit more, e.g. some motivating examples and improved outputs, and share your findings, we'd love to hear them!

[ZevEisenberg]

I hear the bottom is about to fall out of the macros market 😉