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/phpunit/tests/basic_test.php

@@ -37,7 +37,7 @@ defined('MOODLE_INTERNAL') || die();
 class core_phpunit_basic_testcase extends basic_testcase {
     protected $testassertexecuted = false;
 
-    protected function setUp() {
+    protected function setUp(): void {
         parent::setUp();
         if ($this->getName() === 'test_setup_assert') {
             $this->assertTrue(true);