specs: move legacy forward-char tests to specl.

Close #2
* tests/forward-char.el, tests/forward-char.output,
tests/interactive/backward-char.el,
tests/interactive/backward-char.output,
tests/regression/forward-char_at_end.el,
tests/regressino/forward-char_at_end.output: Move from here...
* lib/zmacs/specs/zlisp_move_spec.yaml (forward-char): ...to
here, and improve in line with backward-char specifications.
* lib/zemacs/zlisp/move.zl (forward-char): Use (interactive "p")
to check argument type, and output the correct diagnostic for
moving beyond the boundaries of the buffer.

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