test: fix incorrect property name $_original

Author: kenjis

tests/system/Entity/EntityTest.php

@@ -1185,7 +1185,7 @@ protected function getMappedEntity()
                 'foo'    => null,
                 'simple' => null,
             ];
-            protected $_original = [
+            protected $original = [
                 'foo'    => null,
                 'simple' => null,
             ];
@@ -1215,7 +1215,7 @@ protected function getSwappedEntity()
                 'foo' => 'foo',
                 'bar' => 'bar',
             ];
-            protected $_original = [
+            protected $original = [
                 'foo' => 'foo',
                 'bar' => 'bar',
             ];
@@ -1234,7 +1234,7 @@ protected function getSimpleSwappedEntity()
                 'foo' => 'foo',
                 'bar' => 'bar',
             ];
-            protected $_original = [
+            protected $original = [
                 'foo' => 'foo',
                 'bar' => 'bar',
             ];
@@ -1263,7 +1263,7 @@ protected function getCastEntity($data = null): Entity
                 'twelfth'    => null,
                 'thirteenth' => null,
             ];
-            protected $_original = [
+            protected $original = [
                 'first'      => null,
                 'second'     => null,
                 'third'      => null,
@@ -1313,7 +1313,7 @@ protected function getCastNullableEntity()
                 'integer_0'             => null,
                 'string_value_not_null' => 'value',
             ];
-            protected $_original = [
+            protected $original = [
                 'string_null'           => null,
                 'string_empty'          => null,
                 'integer_null'          => null,
@@ -1341,7 +1341,7 @@ protected function getCustomCastEntity()
                 'third'  => null,
                 'fourth' => null,
             ];
-            protected $_original = [
+            protected $original = [
                 'first'  => null,
                 'second' => null,
                 'third'  => null,