MDL-75111 phpunit: Move tests to use correct names and ns (take#4)

MDL-75111 phpunit: Move tests to use correct names and ns (take#4)

Applied the following changes to various testcase classes:

- Namespaced with component[\level2-API]
- Moved to level2-API subdirectory when required.
- Fixed incorrect use statements with leading backslash.
- Remove file phpdoc block
- Remove MOODLE_INTERNAL if not needed.
- Changed code to point to global scope when needed.
- Fix some relative paths and comments here and there.
- All them passing individually.
- Complete runs passing too.

Special mention to:

- The following task tests have been moved within the level2 directory:
  - \core\adhoc_task_test => \core\task\adhoc_task_test
  - \core\scheduled_task_test => \core\task\scheduled_task_test
  - \core\calendar_cron_task_test => \core\task\calendar_cron_task_test
  - \core\h5p_get_content_types_task_test => \core\task\h5p_get_content_types_task_test
  - \core\task_database_logger_test => \core\task\database_logger_test
  - \core\task_logging_test => \core\task\logging_test

- The following event tests have been moved within level2 directory:
  - \core\event_context_locked_test => \core\event\context_locked_test
  - \core\event_deprecated_test => \core\event\deprecated_test
  - \core\event_grade_deleted_test => \core\event\grade_deleted_test
  - \core\event_profile_field_test => \core\event\profile_field_test
  - \core\event_unknown_logged_test => \core\event\unknown_logged_test
  - \core\event_user_graded_test => \core\event\user_graded_test
  - \core\event_user_password_updated_test => \core\event\user_password_updated_test

- The following output tests have been moved within level2 directory:
  - \core\mustache_template_finder_test => \core\output\mustache_template_finder_test
  - \core\mustache_template_source_loader_test => \core\output\mustache_template_source_loader_test
  - \core\output_mustache_helper_collection_test => \core\output\mustache_helper_collection_test

- The following tests have been moved to their correct tests directories:
  - lib/tests/time_splittings_test.php => analytics/tests/time_splittings_test.php

- All the classes and tests under lib/filebrowser and lib/filestorage
  belong to core, not to core_files. Some day we should move
  them to their correct subsystem.
- All the classes and tests under lib/grade belong to core, not
  to core_grades. Some day we should move them to their correct
  subsystem.
- The core_grades_external class and its \core\grades_external_test
  unit test should belong to the grades subsystem or, alternatively,
  to \core\external, they both should be moved together.
- The core_grading_external class and its \core\grading_external_test
  unit test should belong to the grading subsystem or, alternatively,
  to \core\external, they both should be moved together.
- The \core\message\message and \core\message\inbound (may be others)
  classes, and their associated tests should go to the core_message
  subsystem.
- The core_user class, and its associated tests should go to the
  core_user subsystem.
- The \core\update namespace is plain wrong (update is not valid API)
  and needs action 1) create it or 2) move elsewhere.