MDL-37774 Add unit test to illustrate the problem with the trailing slash

The patch introduces two new tests for moodle1_file_manager::migrate_directory().
The second test with the trailing slash reveals the trouble here.
The expected behaviour is that the method would deal with the trailing
slash. Debugging message for the developer should be displayed though.
MDL-37774 Make moodle1_file_manager::migrate_directory() support trailing slash in rootpath

As the $rootpath is concatenated with the $relpath, we would end with a
path like course_files//file.txt. Such a path is detected as not-valid
later and an error would be thrown.

So the patch makes sure that the trailing slash is cut off and a
debugging for the developer is displayed.
MDL-37774 Do not pass trailing slash if the method does not want it