[mount] Repair test-config files against schema

- can't use *bogus* as a key unless that's allowed by
  the schema -- and it is not.
- can't supply empty config if there is supposed to be
  an object there.

This makes it a little difficult to allow a config-file
that is actually no-configuration-at-all (or only-defaults).
Put in values for *btrfsSwapSubvol* since it isn't a fragile
setting.

src/modules/mount/tests/1.job

@@ -1,6 +1,6 @@
 # SPDX-FileCopyrightText: no
 # SPDX-License-Identifier: CC0-1.0
-bogus: true
+btrfsSwapSubvol: ""
 
 # No configuration needed because the partitions are
 # all filesystems that require no special handling.

src/modules/mount/tests/2.job

@@ -1,5 +1,6 @@
 # SPDX-FileCopyrightText: no
 # SPDX-License-Identifier: CC0-1.0
+btrfsSwapSubvol: ""
 
 # Expect a complaint and a default btrfs layout because the
 # partitions use btrfs

src/modules/mount/tests/3.job

@@ -1,6 +1,6 @@
 # SPDX-FileCopyrightText: no
 # SPDX-License-Identifier: CC0-1.0
-bogus: true
+btrfsSwapSubvol: ""
 
 # No configuration needed because the partitions are
 # all filesystems that require no special handling.