[mount] Add test exercising partial-filtering

src/modules/mount/tests/4.global

+# SPDX-FileCopyrightText: no
+# SPDX-License-Identifier: CC0-1.0
+partitions:
+    - device: "/dev/sdb1"
+      mountPoint: "/"
+      fs: "btrfs"
+    - device: "/dev/sdb2"
+      mountPoint: "/home"
+      fs: "ext4"
+partitionChoices:
+    swap: file

src/modules/mount/tests/4.job

+# SPDX-FileCopyrightText: no
+# SPDX-License-Identifier: CC0-1.0
+
+btrfsSubvolumes:
+    - mountPoint: /
+      subvolume: /@
+    - mountPoint: /home
+      subvolume: /@home
+    - mountPoint: /var/cache
+      subvolume: /@cache
+    - mountPoint: /var/log
+      subvolume: /@log