zpaq: use assertion instead of test return value

Closes Homebrew/homebrew#29483.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/zpaq.rb

@@ -21,6 +21,6 @@ class Zpaq < Formula
     zpaq = bin/"zpaq"
     system zpaq, "a", archive, "#{include}/libzpaq.h"
     system zpaq, "t", archive
-    archive.read(4) == "7kSt"
+    assert_equal "7kSt", archive.read(4)
   end
 end