runtime: skip TestArenaCollision on failed reservation
If TestArenaCollision cannot reserve the address range it expects to
reserve, it currently fails somewhat mysteriously. Detect this case
and skip the test. This could lead to test rot if we wind up always
skipping this test, but it's not clear that there's a better answer.
If the test does fail, we now also log what it thinks it reserved so
the failure message is more useful in debugging any issues.