There seems to be another manifestation of bug #20021, where some of the
compaction tests are failing on i686 for unrelated PR's because of fake
"live" references to moved objects on the machine stack.

We _could_ solve this by counting how many objects are pinned during
compaction, but doing that involves pushing down the mark & pin bitset
merge into gc_compact_plane and out of gc_compact_page, which I thought
was pretty ugly.

Now that we've solved bug #20022 though, we're able to compact
arbitrarily many objects with GC.verify_compaction_references, so the
number of objects we're moving is now 50,000 instead of 500. Since
that's now much larger than the number of objects likely to be pinned, I
think it's safe enough to just add a fudge-factor to the tests.

Any _other_ change in GC.verify_compaction_references that breaks
compaction is now highly likely to break the assertion by more than 10
objects, since it's operating on so many more in the first place.

[Bug #20021]