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...

cohort/tests/privacy_test.php

@@ -42,7 +42,7 @@ class core_cohort_testcase extends provider_testcase {
     /**
      * Basic setup for these tests.
      */
-    public function setUp() {
+    public function setUp(): void {
         $this->resetAfterTest(true);
     }