MDL-81698 phpunit: Apply various fixes towards 1by1 execution

The changes here are heterogeneous:
- Include stuff that is not available (other test has included it).
  Sometimes local to a unit test, others in setupBeforeClass() or
  globally, ... depends on every case.
- Rename some tests (namespaces, test name, ...) towards getting it
  running.
- Amend small bits here and there.

Important note: I've left any "cosmetic" warning out from the
changes, only a few errors (like long array syntax) have been fixed.

lib/tests/completionlib_test.php

@@ -1690,7 +1690,8 @@ class completionlib_test extends advanced_testcase {
      * @covers \aggregate_completions
      */
     public function test_aggregate_completions() {
-        global $DB;
+        global $DB, $CFG;
+        require_once($CFG->dirroot.'/completion/criteria/completion_criteria_activity.php');
         $this->resetAfterTest(true);
         $time = time();