muscles: fix incorrect decoding of $

Bug introduced in 458171e6df.
https://lists.gnu.org/archive/html/bison-patches/2013-11/msg00009.html

Reported by Ahcheong Lee.
https://lists.gnu.org/r/bug-bison/2020-03/msg00010.html

* src/muscle-tab.c (COMMON_DECODE): "$" is coded as "$][", not "$[][".
* tests/input.at ("%define" enum variables): Check that case.

src/muscle-tab.c

@@ -292,7 +292,6 @@ muscle_location_grow (char const *key, location loc)
 
 #define COMMON_DECODE(Value)                                    \
   case '$':                                                     \
-    ++(Value); aver (*(Value) == '[');                          \
     ++(Value); aver (*(Value) == ']');                          \
     ++(Value); aver (*(Value) == '[');                          \
     obstack_sgrow (&muscle_obstack, "$");                       \

src/system.h

@@ -209,10 +209,10 @@ typedef size_t uintptr_t;
 
 /* Output Str both quoted for M4 (i.e., embed in [[...]]), and escaped
    for our postprocessing (i.e., escape M4 special characters).  If
-   Str is empty (or NULL), output "[]" instead of "[[]]" as it make M4
-   programming easier (m4_ifval can be used).
+   Str is empty (or NULL), output "[]" instead of "[[]]" as it makes
+   M4 programming easier (m4_ifval can be used).
 
-   For instance "[foo]" -> "[[@{foo@}]]", "$$" -> "[[$][$][]]". */
+   For instance "[foo]" -> "[[@{foo@}]]", "$$" -> "[[$][$][]]".  */
 
 # define obstack_quote(Obs, Str)                \
   do {                                          \