MDL-79717 phpunit: ensure unique data provider keys in tests.

Duplicate data provider keys were overwriting and/or duplicating
one another, leading to some cases being skipped.

Other "duplicate array key" errors were picked up by `phpcs` in
this dragnet across all tests, which have also been fixed.

lib/tests/moodlelib_test.php

@@ -25,7 +25,7 @@ namespace core;
  * @author     [email protected]
  * @author     [email protected]
  */
-class moodlelib_test extends \advanced_testcase {
+final class moodlelib_test extends \advanced_testcase {
 
     /**
      * Define a local decimal separator.
@@ -858,13 +858,15 @@ class moodlelib_test extends \advanced_testcase {
             '-13.5'                          => '',
             '0.2'                            => '',
             ''                               => '',
-            null                             => '',
         );
 
         foreach ($testvalues as $testvalue => $expectedvalue) {
             $actualvalue = clean_param($testvalue, PARAM_TIMEZONE);
             $this->assertEquals($expectedvalue, $actualvalue);
         }
+
+        // Test for null.
+        $this->assertEquals('', clean_param(null, PARAM_TIMEZONE));
     }
 
     /**
@@ -3575,9 +3577,9 @@ EOF;
 
     /**
      * Data provider for test_generate_confirmation_link
-     * @return Array of confirmation urls and expected resultant confirmation links
+     * @return array Confirmation urls and expected resultant confirmation links
      */
-    public function generate_confirmation_link_provider() {
+    public static function generate_confirmation_link_provider(): array {
         global $CFG;
         return [
             "Simple name" => [
@@ -3645,7 +3647,7 @@ EOF;
                 "confirmationurl" => "http://moodle.org/ext.php?with=some&param=eters",
                 "expected" => "http://moodle.org/ext.php?with=some&param=eters&data=/many_-%2E%40characters%40_%40-%2E%2E-%2E%2E"
             ],
-            "Custom external confirmation url with parameters" => [
+            "Custom external confirmation url with parameters (again)" => [
                 "username" => "many_-.@characters@[email protected]..",
                 "confirmationurl" => "http://moodle.org/ext.php?with=some&data=test",
                 "expected" => "http://moodle.org/ext.php?with=some&data=/many_-%2E%40characters%40_%40-%2E%2E-%2E%2E"