Use a less common port than 4567 in script/test

If someone already has a Sinatra app spun up in development environment,
`script/test` will fail because it will try to use the same port.

Use port 3999 for testing because it's less likely to be used by another app.

Author: mislav

script/server

@@ -1,4 +1,4 @@
 #!/usr/bin/env bash
 set -e
 
-exec bundle exec ruby ./test/app.rb
+exec bundle exec ruby ./test/app.rb "$@"

script/test

@@ -1,8 +1,8 @@
 #!/usr/bin/env bash
 set -e
 
-port=4567
-script/server &>/dev/null &
+port=3999
+script/server -p "$port" &>/dev/null &
 pid=$!
 
 trap "kill $pid" EXIT INT