malloc: Test various special cases related to allocation failures

malloc: Test various special cases related to allocation failures

This test case exercises unusual code paths in allocation functions,
related to allocation failures.  Specifically, the test can reveal
the following bugs:

(a) calloc returns non-zero memory on fallback to sysmalloc.
(b) calloc can self-deadlock because it fails to release
    the arena lock on certain allocation failures.
(c) pvalloc can dereference a NULL arena pointer.

(a) and (b) appear specific to a faulty downstream backport.
(c) was fixed as part of commit 10ad46bc6526edc5c7afcc57112da96917ff3629.

The test for (a) was inspired by a reproducer supplied by Jeff Layton.