…almost exclusively background GCs.

The problem is that we turn off the gradual_decommit_in_progress_p flag and rely on the distribute_free_regions flag to turn it back on. If we only trigger a BGC however, distribute_free_regions won't get called and the flag will stay off, despite the global_regions_to_decommit list being non-empty.

This will cause regions to accumulate in the global_regions_to_decommit list, and eventual will cause the process to run out of memory.

…test for the no gc region case in decommit_step.