Add some whitespace variations to test 15.

This shows two minor failures in our current parsing (resulting in
whitespace-only changes, oso not that significant):

  1. We are inserting extra whitespace between tokens not originally
     separated by whitespace in the replacement list of a macro
     definition.

  2. We are swallowing whitespace separating tokens in the general
     content.

tests/015-define-object-with-parens.c

@@ -1,2 +1,4 @@
 #define foo ( ) 1
 foo()
+#define bar () 2
+bar( )