Use check-expand-* instead of check-equal? for threading tests

I opted out of using one `module+` block per macro because it got too crowded IMHO.

Let me know if you think I overdid it with `test-threader`. (I wanted to test arity 1 only once and figured it would be right to do it on something else than the real threading macros. OTOH, `threading-syntax-parser` is an implementation detail which begs the question of white vs. black box testing.)

\*sigh\* Testing is hard.