test: specialize OOM check for AIX

Assumption that if memory can be malloc()ed it can be used is not true
on AIX. Later access of the allocated pages can trigger SIGKILL if there
are insufficient VM pages.

Use psdanger() to better estimate available memory.

Fixes: https://github.com/nodejs/build/issues/1849

More info:
- https://www.ibm.com/support/knowledgecenter/en/ssw_aix_71/generalprogramming/sys_mem_alloc.html
- https://www.ibm.com/support/knowledgecenter/en/ssw_aix_71/p_bostechref/psdanger.html

Related to:
- https://github.com/nodejs/build/issues/1820#issuecomment-505998851
- https://github.com/nodejs/node/pull/28469
- https://github.com/nodejs/node/pull/28516

PR-URL: https://github.com/nodejs/node/pull/28857


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>