MDL-78160 lib: Added class properties that are not declared in dml

In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.

At the test_transaction_with_events() I changed $this->_called into $called local var,
since the $called is used inside the anonymous function I added the third param as
a variable passed by reference so the last assert can get the newest value.

lib/dml/tests/dml_read_slave_test.php

@@ -400,15 +400,15 @@ class dml_read_slave_test extends \base_testcase {
 
             $this->assertNull($DB->get_dbhwrite());
 
-            $this->_called = false;
+            $called = false;
             $transaction = $DB->start_delegated_transaction();
             $now = microtime(true);
 
             $observers = [
                 [
                     'eventname'   => '\core_tests\event\unittest_executed',
-                    'callback'    => function (\core_tests\event\unittest_executed $event) use ($DB, $now) {
-                        $this->_called = true;
+                    'callback'    => function (\core_tests\event\unittest_executed $event) use ($DB, $now, &$called) {
+                        $called = true;
                         $this->assertFalse($DB->is_transaction_started());
 
                         // This condition should always evaluate true, however we need to
@@ -448,7 +448,7 @@ class dml_read_slave_test extends \base_testcase {
             $event->trigger();
             $transaction->allow_commit();
 
-            $this->assertTrue($this->_called);
+            $this->assertTrue($called);
         });
     }
 

lib/dml/tests/fixtures/read_slave_moodle_database.php

@@ -80,14 +80,6 @@ class read_slave_moodle_database extends test_moodle_database {
     protected function commit_transaction() {
     }
 
-    /**
-     * Abort database transaction
-     * @return void
-     */
-    protected function rollback_transaction() {
-        $this->txnhandle = $this->handle;
-    }
-
     /**
      * Query wrapper that calls query_start() and query_end()
      * @param string $sql

lib/dml/tests/fixtures/test_moodle_read_slave_trait.php

@@ -25,6 +25,8 @@
 
 namespace core;
 
+use ReflectionProperty;
+
 /**
  * Read slave helper that exposes selected moodle_read_slave_trait metods
  *
@@ -49,7 +51,11 @@ trait test_moodle_read_slave_trait {
         fputs($ro, 'ro');
 
         $this->prefix = 'test_'; // Default, not to leave empty.
-        $this->wantreadslave = true;
+
+        $rcp = new ReflectionProperty(parent::class, 'wantreadslave');
+        $rcp->setAccessible(true);
+        $rcp->setValue($this, true);
+
         $this->dbhwrite = $rw;
         $this->dbhreadonly = $ro;
         $this->set_db_handle($this->dbhwrite);