gnu: cups: Don't set #:tests? to #:true.

* gnu/packages/cups.scm (cups)[arguments]: Rewrite to strip the #:tests?
argument from the inherited package.

gnu/packages/cups.scm

@@ -354,9 +354,10 @@ (define-public cups
   (package/inherit cups-minimal
     (name "cups")
     (arguments
-     (substitute-keyword-arguments (package-arguments cups-minimal)
-       ((#:tests? _ #t)
-        #t)
+     (substitute-keyword-arguments
+       (strip-keyword-arguments
+         '(#:tests?)
+         (package-arguments cups-minimal))
        ((#:configure-flags flags #~'())
         #~(append #$flags
                   (list "--with-languages=all"))) ; no ‘=all’ means none(!)