psftools: use test assertion

Formula/psftools.rb

@@ -21,7 +21,7 @@ class Psftools < Formula
     # The zip file has a fon in it, use fon2fnts to extrat to fnt
     Pc8x8Font.new.brew do
       system "#{bin}/fon2fnts", "pc8x8.fon"
-      raise unless File.exist? "PC8X8_9.fnt"
+      assert File.exist?("PC8X8_9.fnt")
     end
   end
 end