[antlir2][container_subtarget] add check for 'systemctl is-system-running'

Summary:
This proves that systemd is actually up and running as pid1 instead of just
getting a bash prompt.

___

Test Plan:
```
❯ buck2 test fbcode//antlir/antlir2/container_subtarget/tests:test -- test_rooted_boot_exit_code test_rootless_boot_exit_code
Buck UI:    https://www.internalfb.com/buck2/435d727f-a0e9-4ff9-b4c9-8329752aaf87
Tests finished: Pass 1. Fail 0. Timeout 0. Fatal 0. Skip 0. Omit 0. Infra Failure 0. Build failure 0
```
https://www.internalfb.com/intern/testinfra/testrun/9851624370074018

Differential Revision: D113494558

fbshipit-source-id: 53557cfbf0d7018b419ad63bf8453659688b71aa

Author: vmagro

antlir/antlir2/container_subtarget/tests/test.rs

@@ -65,6 +65,10 @@ fn test_rooted_boot_exit_code() {
         .expect("failed to spawn");
     p.exp_regex("[^\n\r](.*?)# ")
         .expect("didn't get bash prompt");
+    p.send_line("systemctl is-system-running")
+        .expect("failed to write shell command line");
+    p.exp_regex("running")
+        .expect("didn't get 'running' response from systemctl is-system-running");
     p.send_line("exit 42")
         .expect("failed to write shell command line");
     let status = p.process.wait().expect("failed to wait for process");