tests: fix test_hwtimer_spin

Actually test hwtimer_spin, not hwtimer_arch*

`hwtimer_wait` calls `hwtimer_spin` only for delays `<= 6`, that's why a
pure test_hwtimer_spin test should not pass values `>6`.
Also decrement delay in steps of 1.
tests: add test_hwtimer_wait

It contains the old test functionality of test_hwtimer_spin.
Merge pull request #1044 from LudwigOrtmann/issue_715

tests: fix test_hwtimer_spin