Adding attribute to the post.php form tag in wp-admin

The form is generated in file wp-admin/edit-form-advanced.php. The relevant code starts at line 410 (WP version 4.0.1). Inside the <form> tag an action is fired: <?php do_action( 'post_edit_form_tag', $post ); ?> so it should be easy to use this action by hooking some small function to it from functions.php:

function post_form_novaidate() {
    echo ' novalidate="novalidate" ';
}
add_action( 'post_edit_form_tag', 'post_form_novaidate' );

Didn’t test, but should work.