testValidateMultipleOf() requires "bcmath" ext

In v8.12.0 the validation rule 'multiple_of'
started calling global function bcmod() which
is part of the "bcmath" extension.

Add @requires phpdoc to the 'multiple_of' test
method for better feedback if the PHP extension
is missing in the environment.

tests/Validation/ValidationValidatorTest.php

@@ -1857,8 +1857,9 @@ public function testValidateMax()
      * @param bool $passes
      *
      * @dataProvider multipleOfDataProvider
+     * @requires extension bcmath
      */
-    public function testValidateMutlpleOf($input, $allowed, $passes)
+    public function testValidateMultipleOf($input, $allowed, $passes)
     {
         $trans = $this->getIlluminateArrayTranslator();
         $trans->addLines(['validation.multiple_of' => 'The :attribute must be a multiple of :value'], 'en');