Fix compatibility with nette/forms 3.3

[slischka]

Summary

• keep Multiplier::getControls() compatible with the Iterator return type while accepting the iterable returned by nette/forms 3.3
• add a regression test for the public return type
• update testing-helpers and keep the test-only nette/application dependency below 3.3 because testing-helpers 4.1.1 has an incompatible LatteFactory::create() signature

Verification

• test suite passes with nette/forms 3.3.0
• test suite and PHPStan pass with the lowest supported dependencies (nette/forms 3.2.4)
• CodeSniffer passes