tests: Gracefully skip zlib test when zlib is missing.

* tests/zlib.scm: Use 'test-skip' instead of (exit 77)
when (zlib-available?) returns false.

tests/zlib.scm

 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2016, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -26,11 +26,10 @@ (define-module (test-zlib)
 
 ;; Test the (guix zlib) module.
 
-(unless (zlib-available?)
-  (exit 77))
-
 (test-begin "zlib")
 
+(unless (zlib-available?)
+  (test-skip 1))
 (test-assert "compression/decompression pipe"
   (let ((data (random-bytevector (+ (random 10000)
                                     (* 20 1024)))))