QDoc: Cover signal declarations in QML code blocks

Signal declarations in highlighted QML blocks must survive parser
API changes. Using a property-only AST accessor for a signal can
abort QDoc during a documentation build.

Add a minimal \qml block containing a signal declaration to the
output-validator fixtures and check its generated markup. This
catches the invalid AST access before a documentation build.

Task-number: QTBUG-151041
Change-Id: I7a19d7acd0dee1a9ffbc70e63e22c500eddc93dc
Reviewed-by: David Boddie <david.boddie@qt.io>