test: move flaky test to test/pummel

Move sequential/test-crypto-timing-safe-equal-benchmarks to test/pummel
because it fails for me locally quite frequently and because it takes
about five or six seconds to complete, which is too long for a test in
test/sequential.

Fixes: https://github.com/nodejs/node/issues/8744
PR-URL: https://github.com/nodejs/node/pull/9241


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: not-an-aardvark <not-an-aardvark@users.noreply.github.com>