toaster-tests: fix erroneous message when test fails

toaster-tests: fix erroneous message when test fails

When one of the layer details tests fails (as it occasionally
does, if running on a machine under heavy load, due to sync issues),
the error message shown is misleading, as it is something like:

"Expected 'This was imported' in ['This was imported', ...]"

The string 'This was imported' is in the list shown in the message,
but the message suggests it isn't.

This is because the test compares the string with one list, but
then uses a different list in the fail message if the comparison
fails.

Fix the list shown in the message about the test failing.

Signed-off-by: Elliot Smith <elliot.smith@intel.com>