Micro-optimize gc scavenge.

Instead of putting the amount by which to increment the pointer and
then adding it at the end of the loop, increment the pointer
directly. Makes it slightly faster when compiled with GCC.
slep-non-consing test: prevent from sleeping too much.

Some platforms can't sleep for short amounts of time, add a
sb-ext:with-timeout around it and don't report it as a failure, since
there's basically nothing that can be fixed in such cases.