storage: restricted probes must gather fs info

In ubuntu-desktop-installer/issues/1772, a user formatted an existing
partition as ext4, which triggered the ESP to be "mounted".  Except this
mount step also formatted the ESP.

Previously, when we ran block probing, the restricted=False version
failed, which caused it to run a restricted=True probe.  The
restricted=True probe looks at block devices, but does not gather
filesystem information.  The esp "mount" check is also willing to format
the partition if it is unformatted.  (We do not have a distinction
between a partition that is unformatted and one for which we have not
obtained filesystem information.)

This user had block probing restricted=False fail due to LP: #2012722,
where Ventoy was in use and we handled the device mapper entry poorly.
While recreating the failure case, simply retesting with a build with
the fix for LP: #2012722 is enough to avoid this problem.  This is
because the restricted=False probe passes, which means filesystem info
is gathered, which means the mount step doesn't attempt to format it.

There will inevitably be other block probing failure cases for
restricted=False.  restricted=True must not leave people so vulnerable
to a partition reformat.

Gather filesystem information during a restricted=True probe.

subiquity/server/controllers/filesystem.py

@@ -846,7 +846,7 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
     @with_context(name='probe_once', description='restricted={restricted}')
     async def _probe_once(self, *, context, restricted):
         if restricted:
-            probe_types = {'blockdev'}
+            probe_types = {'blockdev', 'filesystem'}
             fname = 'probe-data-restricted.json'
             key = "ProbeDataRestricted"
         else:

subiquity/server/controllers/tests/test_filesystem.py

@@ -72,7 +72,8 @@ class TestSubiquityControllerFilesystem(IsolatedAsyncioTestCase):
 
     async def test_probe_restricted(self):
         await self.fsc._probe_once(context=None, restricted=True)
-        self.app.prober.get_storage.assert_called_with({'blockdev'})
+        expected = {'blockdev', 'filesystem'}
+        self.app.prober.get_storage.assert_called_with(expected)
 
     async def test_probe_os_prober_false(self):
         self.app.opts.use_os_prober = False