system tests: fix systemd tests in proxy environment

Under proxy environment, we need to add
proxy environment variables.

Related to: #15639

Signed-off-by: Toshiki Sonoda <sonoda.toshiki@fujitsu.com>

test/system/250-systemd.bats

@@ -33,7 +33,11 @@ function teardown() {
 
 # Helper to start a systemd service running a container
 function service_setup() {
-    run_podman generate systemd --new $cname
+    run_podman generate systemd \
+               -e http_proxy -e HTTP_PROXY \
+               -e https_proxy -e HTTPS_PROXY \
+               -e no_proxy -e NO_PROXY \
+               --new $cname
     echo "$output" > "$UNIT_FILE"
     run_podman rm $cname