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 its parent processes.

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

PR-URL: https://github.com/nodejs/node/pull/29054
Backport-PR-URL: https://github.com/nodejs/node/pull/29599


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>