fix: initialize num_ipv4_tuples variable (#10712)

I believe this test may be flaky because a declared variable in Bash is
perhaps not necessarily initialised.

Resolves: #10711

scripts/tests/download-roaming-network.sh

@@ -50,7 +50,7 @@ readarray -t flows < <(get_flow_logs "tcp")
 assert_gteq "${#flows[@]}" 2
 
 declare -A unique_src_tuples
-declare -i num_ipv4_tuples
+declare -i num_ipv4_tuples=0
 
 for flow in "${flows[@]}"; do
     # All flows should have same inner_dst_ip