Add sleep to nginx system test so it will pass under valgrind.

Without this sleep, running `test/run_tests.sh` with `TEST_NATIVE_FETCHER=true TEST_SERF_FETCHER=false USE_VALGRIND=true` fails. Running outside of valgrind it passed before though.
It looks like $OUT will always be the same before and after this `sleep()`. But it would affect timing between two test_optimize_for_bandwidth() calls.