specs: move legacy {beginning,end}-of-line tests to specl.

* tests/beginning-of-line.el, tests/beginning-of-line.output,
tests/end-of-line.el, tests/end-of-line.output: Remove.
* lib/zemacs/specs/zlisp_move_spec.yaml (beginning-of-line)
(end-of-line): More comprehensive examples to specify behaviour
of these lisp functions.
* lib/zemacs/zlisp/move.zl (beginning-of-line, end-of-line):
Use `(interactive "p")` for argument checking, and rewrite as
clients of new lib/zile/buffer.lua:forward_line () to satisfy
specified behaviours.
* lib/zemacs/zlisp/lisp.zl (evaluate-expression): Until all lisp
calls have correct return values, temporarily treat `true` return
as equivalent to `nil` to match Emacs.
Add FIXME comment to come back and fix this later.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>