While this change is not 100% required now, it's good habit
and we are checking for it since Moodle 4.4.
All the changes in this commit have been applied automatically
using the moodle.PHPUnit.TestReturnType sniff and are, exclusively
adding the ": void" return types when missing.
This issue just goes over all the currently incorrect
namespaces in test cases and:
1. Change the namespace to the correct one.
2. Move/rename it to correct location if needed (level 2 and down).
3. Remove not needed MOODLE_INTERNAL check when possible.
4. Remove file phpdoc when the file only has one class.
5. Make small adjustments in case the change of namespace requires it.