xds interop: Fix buildscripts not continuing on a failed test suite (#9833)

Apparently there's a difference between bash 3 and bash 4.
OSX comes with bash 3 out-of-box, so for whoever wrote this logic
it "worked on my machine".

The `((` construct returns a 0 exit code if the value is non-zero.
Since the value starts at 0 and we do a post-increment,
it will always fail the first time.
Changing it to a pre-increment should fix it.

buildscripts/kokoro/psm-security.sh

@@ -177,7 +177,7 @@ main() {
   local failed_tests=0
   test_suites=("baseline_test" "security_test" "authz_test")
   for test in "${test_suites[@]}"; do
-    run_test $test || (( failed_tests++ ))
+    run_test $test || (( ++failed_tests ))
   done
   echo "Failed test suites: ${failed_tests}"
   if (( failed_tests > 0 )); then

buildscripts/kokoro/xds_k8s_lb.sh

@@ -178,7 +178,7 @@ main() {
   local failed_tests=0
   test_suites=("api_listener_test" "change_backend_service_test" "failover_test" "remove_neg_test" "round_robin_test" "affinity_test" "outlier_detection_test" "custom_lb_test")
   for test in "${test_suites[@]}"; do
-    run_test $test || (( failed_tests++ ))
+    run_test $test || (( ++failed_tests ))
   done
   echo "Failed test suites: ${failed_tests}"
   if (( failed_tests > 0 )); then