Tests: replace assert[Not]InternalType( 'resource', $var ) with `as…

…sertIs[Not]Resource()`

The `assertInternalType()` and `assertNotInternalType()` methods were soft deprecated in PHPUnit 7.5, hard deprecated in PHPUnit 8.0 and the functionality was removed in PHPUnit 9.0.

The more specific `assertIsArray()`, `assertIsNotArray()`, `assertIsBool()`, `assertIsNotBool()`, `assertIsFloat()`, `assertIsNotFloat()`, `assertIsInt()`, `assertIsNotInt()`, `assertIsNumeric()`, `assertIsNotNumeric()`, `assertIsObject()`, `assertIsNotObject()`, `assertIsResource()`, `assertIsNotResource()`, `assertIsString()`,
`assertIsNotString()`, `assertIsScalar()`, `assertIsNotScalar()`, `assertIsCallable()`, `assertIsNotCallable()`, `assertIsIterable()`, `assertIsNotIterable()` methods were introduced as replacements in PHPUnit 7.5.

Ref:
* https://github.com/sebastianbergmann/phpunit/blob/7.5.20/ChangeLog-7.5.md#750---2018-12-07
* sebastianbergmann/phpunit#3368

tests/phpunit/tests/image/functions.php

@@ -302,10 +302,10 @@ public function test_load_directory() {
 
 		// First, test with deprecated wp_load_image function.
 		$editor1 = wp_load_image( DIR_TESTDATA );
-		$this->assertNotInternalType( 'resource', $editor1 );
+		$this->assertIsNotResource( $editor1 );
 
 		$editor2 = wp_get_image_editor( DIR_TESTDATA );
-		$this->assertNotInternalType( 'resource', $editor2 );
+		$this->assertIsNotResource( $editor2 );
 
 		// Then, test with editors.
 		$classes = array( 'WP_Image_Editor_GD', 'WP_Image_Editor_Imagick' );