Use 'level' instead of 'child' 'grandchild' 'great grandchild' in menu_test and associated tests

Problem/Motivation

See discussion on https://git.drupalcode.org/project/drupal/-/merge_requests/4608/diffs#no... and 📌 Restrict access to empty top level administration pages for overview controller Fixed . We could use level 1/2/3 instead of child / grandchild / great grandchild etc. in both the menu_test module as well as the variable names in test coverage.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet