Re-add slow tests to gke-ci-reboot filter

Reboot tests are already slow, should only run fast tests
in this job.

hack/jenkins/e2e.sh

@@ -742,6 +742,7 @@ case ${JOB_NAME} in
           ${GKE_DEFAULT_SKIP_TESTS[@]:+${GKE_DEFAULT_SKIP_TESTS[@]}} \
           ${REBOOT_SKIP_TESTS[@]:+${REBOOT_SKIP_TESTS[@]}} \
           ${GCE_FLAKY_TESTS[@]:+${GCE_FLAKY_TESTS[@]}} \
+          ${GCE_SLOW_TESTS[@]:+${GCE_SLOW_TESTS[@]}} \
           )"}
     ;;