MDL-81521 core: Update all possibly data providers to be static

Note: Some data providers could not be automatically be converted to
being static. These will be handled in a separate issue.

message/tests/api_test.php

@@ -1840,7 +1840,7 @@ final class api_test extends \advanced_testcase {
     * This provides sets of data to for testing.
     * @return array
     */
-   public function get_conversations_mixed_provider() {
+   public static function get_conversations_mixed_provider(): array {
        return array(
             'Test that conversations with messages contacts is correctly ordered.' => array(
                 'users' => array(
@@ -5834,7 +5834,7 @@ final class api_test extends \advanced_testcase {
     /**
      * Data provider for test_get_conversation_counts().
      */
-    public function get_conversation_counts_test_cases() {
+    public static function get_conversation_counts_test_cases(): array {
         $typeindividual = api::MESSAGE_CONVERSATION_TYPE_INDIVIDUAL;
         $typegroup = api::MESSAGE_CONVERSATION_TYPE_GROUP;
         $typeself = api::MESSAGE_CONVERSATION_TYPE_SELF;