Unit test core_update_code_manager_testcase::test_list_plugin_folder_files() fails on Windows

As reported by stronk7 in MDL-49329 comment
https://tracker.moodle.org/browse/MDL-49329?focusedCommentId=377533&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-377533 there is one new unit test failing on Windows.

Windows use backslash as the directory separator so the full paths comparison fails. After several prototypes, we agreed that the best approach is to normalize the paths returned by the list_plugin_folder_files() method so that it always uses forward slashes on all platforms.