[COMMIT] Test that all listed langenv input methods exist.

[Stephen J. Turnbull]

QUERY

Aidan Kehoe writes:

 > +         ;; on OS X. LATIN CAPITAL LETTER D WITH STROKE does decompose.
                                                            ^
"does not decompose"? --------------------------------------+

BTW, I think the right way to handle the input methods is in the
packages, despite your comment that

 > 	The other input methods require changes to mule-category.el that
 > 	21.4 doesn't have, and as such supporting them in the packages is
 > 	mostly impossible.

I think we can simply define those changes as a feature, either with
fboundp or with featurep, and require it.