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...

lib/tests/task_database_logger_test.php

@@ -43,7 +43,7 @@ class task_database_logger_testcase extends advanced_testcase {
     /**
      * Setup to backup the database before mocking.
      */
-    public function setUp() {
+    public function setUp(): void {
         global $DB;
 
         $this->DB = $DB;
@@ -52,7 +52,7 @@ class task_database_logger_testcase extends advanced_testcase {
     /**
      * Tear down to unmock the database where it was mocked.
      */
-    public function tearDown() {
+    public function tearDown(): void {
         global $DB;
 
         $DB = $this->DB;