specs: move legacy backward-char tests to specl.

* tests/backward-char.el, tests/backward-char.output: Remove.
* tests/zile-only/interactive/backward-char.el,
tests/zile-only/interactive/backward-char.output: Remove.
* lib/zmacs/specs/zlisp_move_spec.yaml: New file, with improved
specs for backward-char behaviour.
* lib/zmacs/zpecs/specs.mk (specl_SPECS): Add
lib/zmacs/specs/zlisp_move_spec.yaml.
* lib/zile/minibuf.lua (trim_messages): Accept a new, optional,
`msg` parameter, defaulting to `minibuf_contents`.
(minibuf_error): Report interactive errors to *Messages* buffer,
prefixed with 'call-interactively: ', like Emacs. This is reqd
for new specs above to pass.

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