uniq tests: Make uniq_test:stdout more reliable

When running regression tests in paralle, this one occasionally fails
because uniq exits with status 0. I believe this is because the test is
a bit racy: it assumes that true(1) will exit before uniq writes to
standard out.

Just sleep for a bit to give the other end of the pipe to exit.

Reviewed by: des
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45534