redis: test for var path existence.

Closes #11458.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/redis.rb

@@ -80,5 +80,6 @@ class Redis < Formula
 
   test do
     system bin/"redis-server", "--test-memory", "2"
+    %w[run db/redis log].each { |p| assert (var/p).exist?, "#{var/p} doesn't exist!" }
   end
 end