Add clsas for MD_STD_STRINGS more obscure string operations

tests/MD_STD_Test.php

@@ -739,6 +739,7 @@ public function test_strpos_multi(string $haystack, array $needles, int $expecte
             self::assertEmpty(MD_STD::strpos_multi($haystack, $needles));
         }
         else {
+            self::assertNotEmpty(MD_STD::strpos_multi($haystack, $needles));
             self::assertEquals($expected, MD_STD::strpos_multi($haystack, $needles)['position']);
         }