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...

privacy/classes/tests/provider_testcase.php

@@ -39,7 +39,7 @@ abstract class provider_testcase extends \advanced_testcase {
     /**
      * Test tearDown.
      */
-    public function tearDown() {
+    public function tearDown(): void {
         \core_privacy\local\request\writer::reset();
     }
 

privacy/tests/manager_test.php

@@ -45,7 +45,7 @@ class privacy_manager_testcase extends advanced_testcase {
     /**
      * Test tearDown.
      */
-    public function tearDown() {
+    public function tearDown(): void {
         \core_privacy\local\request\writer::reset();
     }
 

privacy/tests/writer_test.php

@@ -45,7 +45,7 @@ class writer_test extends advanced_testcase {
      * Ensure that the writer is cleared away as appropriate after each
      * test.
      */
-    public function tearDown() {
+    public function tearDown(): void {
         writer::reset();
     }