assert: ensure posix compliance, add tests for such

assert: ensure posix compliance, add tests for such

Fix assert.c so that even the fallback
case conforms to POSIX, although not exactly the same as
the default case so a test can tell the difference.

Add a test that verifies that abort is called, and that the
message printed to stderr has all the info that POSIX requires.
Verify this even when malloc isn't usable.

Reviewed-by: Paul Eggert <eggert@cs.ucla.edu>