test_faulthandler: skip test_read_null() on AIX

AIX maps the first page of memory at address zero as valid, read-only.  Reading
NULL is not a fault on AIX. This is utilized by IBM compiler optimizations.
One speculatively can indirect through a pointer which may be null without
first testing if null and defer the test before using the value.
test_time.test_monotonic(): use a longer sleep to try to make the test more reliable