Print form name in form_start function
merged branch agiuliano/2.3 (PR #8585)

This PR was submitted for the 2.3 branch but it was merged into the master branch instead (closes #8585).

Discussion
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Print form name in form_start function

It'd be useful if form_start() prints the form's name.
This way if you want to test it in your page, you can catch the form by `$crawler->filter('form[name=my_form_name]')`

Commits
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221bc15 Print form name in form_start function