style: Adjust test to not emit blank lines.

Previously this test would produce a file containing blank lines between
inputs.

* tests/style.scm ("input labels, modify-inputs and margin comment"):
Remove trailing newlines in replacement strings of 'substitute*'
expression.

tests/style.scm

@@ -355,9 +355,9 @@
 
       (substitute* file
         ((",gmp\\)(.*)$" _ rest)
-         (string-append ",gmp) ;margin comment\n" rest))
+         (string-append ",gmp) ;margin comment" rest))
         ((",acl\\)(.*)$" _ rest)
-         (string-append ",acl) ;another one\n" rest)))
+         (string-append ",acl) ;another one" rest)))
 
       (system* "guix" "style" "-L" directory "-S" "inputs"
                "my-coreutils")