zsh-completion: add test

Closes Homebrew/legacy-homebrew#38055.

Signed-off-by: Xu Cheng <xucheng@me.com>

Formula/zsh-completions.rb

@@ -1,7 +1,7 @@
 class ZshCompletions < Formula
   homepage "https://github.com/zsh-users/zsh-completions"
   url "https://github.com/zsh-users/zsh-completions/archive/0.12.0.tar.gz"
-  sha1 "1c39734328b28c619a55fa52fa42e81c314aeadf"
+  sha256 "770d92749b11b22192595b207208508f8bfa319d5d03210a71bc44c8b9cfa0d5"
 
   head "https://github.com/zsh-users/zsh-completions.git"
 
@@ -25,4 +25,13 @@ def caveats
       chmod go-w /usr/local/share
     EOS
   end
+
+  test do
+    (testpath/".zshrc").write <<-EOS.undent
+      fpath=(#{HOMEBREW_PREFIX}/share/zsh-completions $fpath)
+      autoload -U compinit
+      compinit
+    EOS
+    system "/bin/zsh", "--login", "-i", "-c", "which _ack"
+  end
 end