test/lib: Wait for kernel to free up ports used by iperf3 before reusing them

If we start another server on the same port right away, we might fail
to bind the port. A small delay appears to be needed -- I'm not
entirely sure why at this point.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

test/lib/test

@@ -68,6 +68,8 @@ test_iperf3() {
 		'for i in $(seq 0 '${__procs}'); do rm s${i}.json; done'
 
 	TEST_ONE_subs="$(list_add_pair "${TEST_ONE_subs}" "__${__var}__" "${__bw}" )"
+
+	sleep 3		# Wait for kernel to free up ports
 }
 
 test_one_line() {