selftests/unit/test_utils_partition.py: also treat raised OSError

When skipping tests due to the lack of sudo permissions, OSError
can be raised when the asked binary does not exist and can not be
executed.

In an ideal world, only one "skip" decorator would have to run, and
this extra treatment wouldn't be necessary since there's already a
`missing_binary` being used.  Unfortunately, that's not how they work.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

selftests/unit/test_utils_partition.py

@@ -33,7 +33,7 @@ def cannot_sudo(command):
     try:
         process.run(command, sudo=True)
         False
-    except process.CmdError:
+    except (process.CmdError, OSError):
         return True