test: tweak test_osbuild_mount_failure_msg() for tmt/fc40

The `test_osbuild_mount_failure_msg` currently fails on fc40 when
run in tmt, see:
https://artifacts.dev.testing-farm.io/c6588a82-a2cb-46df-8ca8-85dd809465f2/

This is because the failure output is slightly different between
a container and a VM/real-machine. The test ensures that we capture
the output of mount and present to the user (for easier debugging).
So this commit updates this test once more for the error string
(that part of the error comes directly from the kernels fsconfig).

If we need another update of the string we should reconsider this
test and e.g. just use `testutil.mock_command()` for this. But
for now it's easier to just add this one more failure string.

test/mod/test_util_mnt.py

@@ -57,4 +57,5 @@ def test_osbuild_mount_failure_msg(tmp_path):
             "options": [],
         }
         mnt_service.mount(args)
-    assert re.search(r"special device /dev/invalid-src does not exist|Can't open blockdev.", str(e.value))
+    assert re.search(
+        r"special device /dev/invalid-src does not exist|Can't open blockdev.|Can't lookup blockdev", str(e.value))