machine_core: Add Machine.ws_container runtime detection

We want to introduce proper bootc image testing for cockpit. In that
scenario we want to install the actual rpm, not use the cockpit/ws
container. This conflicts with cockpit-ostree's current assumption that
the centos-9-bootc image always uses the ws container.

Thus make this check dynamic: If cockpit-ws package is installed (check
for `/usr/share/cockpit/static` as that's the same on all distros and a
cheap test), use it, otherwise use the ws container. Keep the
`TEST_SCENARIO=ws-container` check, at least until we update
cockpit-ostree's image preparation to remove cockpit-ws.rpm.

We can't yet run `.execute()` in the Machine constructor, so delay the
detection by making `ws_container` a (cached) property.

machine/machine_core/machine.py

@@ -20,6 +20,7 @@
 import re
 import subprocess
 from collections.abc import Collection, Mapping, Sequence
+from functools import cached_property
 
 from lib.constants import BOTS_DIR, DEFAULT_IDENTITY_FILE, OSTREE_IMAGES
 
@@ -91,9 +92,6 @@ def __init__(
         self.arch = arch
         self.image = image
         self.ostree_image = self.image in OSTREE_IMAGES
-        # start via cockpit/ws container on OSTree or when explicitly requested
-        self.ws_container = self.ostree_image or "ws-container" in os.getenv("TEST_SCENARIO", "")
-
         if ":" in browser:
             self.web_address, _, self.web_port = browser.rpartition(":")
         else:
@@ -103,6 +101,23 @@ def __init__(
         # The Linux kernel boot_id
         self.boot_id = None
 
+    @cached_property
+    def ws_container(self) -> bool:
+        """Return if image uses the cockpit/ws container
+
+        True → ws container; False: cockpit-ws package
+
+        Tests can force using the ws container by setting TEST_SCENARIO=ws-container
+        Otherwise this does runtime detection.
+
+        A project's test image preparation should ensure that only one is installed
+        to avoid confusion.
+        """
+        if "ws-container" in os.getenv("TEST_SCENARIO", ""):
+            return True
+        else:
+            return not self.execute("ls /usr/share/cockpit/static", check=False).strip()
+
     def diagnose(self, tty: bool = True) -> str:
         keys = {
             "ssh_user": self.ssh_user,