Work around legitimate xallocx() failures during testing.

With the removal of subchunk size class infrastructure, there are no
large size classes that are guaranteed to be re-expandable in place
unless munmap() is disabled.  Work around these legitimate failures with
rallocx() fallback calls.  If there were no test configuration for which
the xallocx() calls succeeded, it would be important to override the
extent hooks for testing purposes, but by default these tests don't use
the rallocx() fallbacks on Linux, so test coverage is still sufficient.