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...

auth/email/tests/external_test.php

@@ -43,7 +43,7 @@ class auth_email_external_testcase extends externallib_advanced_testcase {
     /**
      * Set up for every test
      */
-    public function setUp() {
+    public function setUp(): void {
         global $CFG, $DB;
 
         $this->resetAfterTest(true);