Add a test rig for ldisc's local line editing.

I'm about to rewrite it completely, so the first thing I need to do is
to write tests for as much of the functionality as possible, so that I
can check the new implementation behaves in the same ways.

unix/platform.h

@@ -332,6 +332,7 @@ void gtk_setup_config_box(
  */
 #define DEFAULT_CODEPAGE 0xFFFF
 #define CP_UTF8 CS_UTF8                /* from libcharset */
+#define CP_437 CS_CP437                /* used for test suites */
 
 #define strnicmp strncasecmp
 #define stricmp strcasecmp