diagnostics: check the styling

Enable checking of styles even when libtextstyle is not installed.

* src/getargs.h, src/getargs.c (style_debug): New.
(getargs_colors): Set it when --style=debug.
* src/complain.c (begin_use_class, end_use_class): Use it.
* tests/diagnostics.at: New.

tests/testsuite.at

@@ -26,6 +26,9 @@ m4_include([named-refs.at])
 # Output file names.
 m4_include([output.at])
 
+# Diagnostics.
+m4_include([diagnostics.at])
+
 # Skeleton support.
 m4_include([skeletons.at])