nixos/tests/podman: replace deprecated os.system in tls-ghostunnel

Seems the shell script ${gen-ca} doesn't have a prefix, and thus requires being
run on a shell.

nixos/tests/podman/tls-ghostunnel.nix

@@ -83,7 +83,7 @@ import ../make-test-python.nix (
     };
 
     testScript = ''
-      import os
+      import subprocess
       import shlex
 
 
@@ -93,7 +93,7 @@ import ../make-test-python.nix (
 
       def cmd(command):
         print(f"+{command}")
-        r = os.system(command)
+        r = subprocess.run(command, shell=True).returncode
         if r != 0:
           raise Exception(f"Command {command} failed with exit code {r}")