test: fix flaky streams test

Use common.platformTimeout() to fix flaky
test-stream2-readable-empty-buffer-no-eofi on Raspberry Pis.

Fixes: https://github.com/nodejs/node/issues/4493
PR-URL: https://github.com/nodejs/node/pull/4516


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M <Snell&lt;jasnell@gmail.com>