This patch corrects the problem by partially backing out changes
introduced in this commit 2 1/2 years ago:

https://git.savannah.gnu.org/cgit/emacs.git/commit/lisp/progmodes/tcl.el?id=cd5bb4bf3dbad8941d25823f398b595b8f0edbb9

The patch removes the customized tcl-beginning-of-defun-function in
favor of relying on the existing lisp mechanism. It preserves the
customized tcl-end-of-defun-function because that is required for some
test cases.

The expected-failing test was marked as succeeding.

This is a minor change. I did not refer to any code outside of emacs
itself in investigating and correcting this bug. I will not be able to
provide additional paperwork.