test: remove flaky designations for tests

Three tests designated as flaky on Linux have not failed on the
continuous integration server in a long time. Removing flaky designation
for these tests.

Fixes: https://github.com/nodejs/node/issues/4446
PR-URL: https://github.com/nodejs/node/pull/4519


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>