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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines