Add regression test for filter

Filters are used for converting an input value (usually a string)
into a domain specific PHP type and they are bound to the validation scope:
https://doc.nette.org/en/forms/validation#toc-modifying-input-values

45ed76a started filtering components
passed to `Container::validate()` to `Control`s, to appease PHPStan.
But `Multiplier` does not actually have `Control`s as direct children
(other than the ‘Add’ `Submitter`s), so it would stop validating and
filtering multiplied controls.

It has been since fixed in a5a7348,
and more properly in the parent commit but let’s add a test so this
does not happen again.

It can be reproduced by removing `|| $component instanceof Container`
from the parent commit.

tests/Unit/MultiplierTest.php

@@ -330,6 +330,50 @@ public function testSendNestedInnerWithDefault()
 		);
 	}
 
+	/**
+	 * Ensure filters work on submit, since they are dependent on properly set valdation scope.
+	 * Regression test for https://github.com/contributte/forms-multiplier/issues/68
+	 */
+	public function testSubmitFilter()
+	{
+		$response = $this->services->form->createRequest(
+			MultiplierBuilder::create()
+				->fields([])
+				->beforeFormModifier(function (Form $form) {
+					$form->addInteger('num');
+				})
+				->multiplierModifier(function (Multiplier $multiplier) {
+					$multiplier->onCreate[] = function (Container $container) {
+						$container->addInteger('mnum');
+					};
+				})
+				->formModifier(function (Form $form) {
+					$form->onSuccess[] = $form->onError[] = $form->onSubmit[] = function () {
+					};
+				})
+				->createForm()
+		)
+			->setPost([
+				'num' => '11',
+				'm' => [
+					['mnum' => '49'],
+				],
+			])->send();
+
+		$this->assertTrue($response->isSuccess());
+		$this->assertSame([
+				'num' => 11,
+				'm' => [
+					['mnum' => 49],
+				],
+			], $response->getValues());
+
+		$dom = $response->toDomQuery();
+
+		$this->assertDomHas($dom, 'input[name="m[0][mnum]"][value="49"]');
+		$this->assertDomNotHas($dom, 'input[name="m[1][mnum]"]');
+	}
+
 	public function testGroup()
 	{
 		$request = $this->services->form->createRequest(