fix TEST_TO_RUN in ngx_pagespeed (#1364)

Eventually we'll convert remote_config_system_test.sh and nginx_system_test.sh
to use run_test, but until then when TEST_TO_RUN is specified we should skip
the tests in those files and just run system/system_test.sh

test/nginx_system_test.sh

@@ -369,6 +369,13 @@ fi
 # wants different ones than we want, so we need to reset our positional args.
 set -- "$PRIMARY_HOSTNAME"
 source $SYSTEM_TEST_FILE
+
+# If we were only asked to run a single test, it ran in SYSTEM_TEST_FILE, so
+# we're done now and should exit.
+if [ -n ${TEST_TO_RUN+x} ]; then
+  check_failures_and_exit
+fi
+
 source $REMOTE_CONFIG_TEST_FILE
 
 # nginx-specific system tests