Right now tilegx is right on the verge of timeout when it runs,
so adding a bit of headroom seems like the right thing; we
see failures when running tests in parallel.
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 10ad46bc65.
The test for (a) was inspired by a reproducer supplied by Jeff Layton.
