test: unskip tests that now pass on AIX

One skipped test remains, it creates very large Buffer objects, triggering the AIX OOM to kill node and it's parent processes.

See: https://github.com/nodejs/build/issues/1849#issuecomment-514414165

Checklist

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