test/users: modify root's home directory in stage test

Since 9071cd0abb the commands in the
org.osbuild.users stage are run using the osbuild.util.chroot.Chroot
context manager.  This includes mounting /proc into the chroot.

The `usermod` command now fails with

  usermod: user root is currently used by process N

when modifying any property of the root user that requires the user to
not be logged in (like the home directory).

Change the home directory of the root user in the stage test to check
for this issue and catch regressions.

test/data/stages/users/b.json

@@ -699,6 +699,16 @@
               }
             }
           }
+        },
+        {
+          "type": "org.osbuild.users",
+          "options": {
+            "users": {
+              "root": {
+                "home": "/var/roothome"
+              }
+            }
+          }
         }
       ]
     }

test/data/stages/users/b.mpp.yaml

@@ -73,3 +73,8 @@ pipelines:
               keys:
                 - ssh-ed25519 11111111111111111111111111111111111111111111111111111111111111111111 onekey
                 - ssh-ed25519 22222222222222222222222222222222222222222222222222222222222222222222 onekey
+      - type: org.osbuild.users
+        options:
+          users:
+            root:
+              home: /var/roothome

test/data/stages/users/diff.json

@@ -32,6 +32,10 @@
     "/home/test/.bash_logout",
     "/home/test/.bash_profile",
     "/home/test/.bashrc",
+    "/var/roothome",
+    "/var/roothome/.bash_logout",
+    "/var/roothome/.bash_profile",
+    "/var/roothome/.bashrc",
     "/var/spool/mail/bothy",
     "/var/spool/mail/elodie",
     "/var/spool/mail/multikey",
@@ -69,13 +73,13 @@
     "/etc/passwd": {
       "content": [
         "sha256:7039379cb97eea23aa2e9276ebc08009d670cec2c4dd5f861b881d6f48a31b0a",
-        "sha256:5eafb41281ddcbadd6ff4ead6b3e89afa8040c697b828b8662d0fc9c0cad7544"
+        "sha256:9c0e9394aae1b6e1c5900b92b5ef4a53cf60b1e57d3bbb2e5f899f960bf63b82"
       ]
     },
     "/etc/passwd-": {
       "content": [
         "sha256:24117042025a62b47c58edf2a4de90032e6daac4d631f4d07894b61dc3b5ff1c",
-        "sha256:02e8a5e49bbd731b50700323f70b0a6d765e2a6e9a1892a6b3e094618f183fdf"
+        "sha256:5eafb41281ddcbadd6ff4ead6b3e89afa8040c697b828b8662d0fc9c0cad7544"
       ]
     },
     "/etc/shadow": {