MDL-67673 phpunit: Fix the return type of template methods

All the setup/teardown/pre/post/conditions template methods
now are required to return void. This was warned with phpunit 7
and now is enforced.

At the same time, fix a few wrong function names,
provider data and param types, return statements...

my/tests/events_test.php

@@ -40,7 +40,7 @@ class dashboard_events_testcase extends advanced_testcase {
     /**
      * Setup often used objects for the following tests.
      */
-    protected function setup() {
+    protected function setUp(): void {
         global $USER;
 
         $this->resetAfterTest();