[DomCrawler] fixed HTML5 form attribute handling XPath query

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/DomCrawler/commit/0a67c88e0eb408f24233775388cd8c3cca286c05#commitcomment-4120595
| License       | MIT
| Doc PR        | no

The XPath query used was not handling descendant form elements correctly, so I fixed it and added tests for it.