refactor(php): use self::markTestIncomplete instead of instance calls (#17824)

It's reported by phpstan as `Dynamic call to static method PHPUnit\Framework\Assert::markTestIncomplete().`.

The method is static, therefore it should not be called from instance context.

samples/client/echo_api/php-nextgen/tests/Model/StringEnumRefTest.php

@@ -77,6 +77,6 @@ class StringEnumRefTest extends TestCase
     public function testStringEnumRef()
     {
         // TODO: implement
-        $this->markTestIncomplete('Not implemented');
+        self::markTestIncomplete('Not implemented');
     }
 }