Stop tests from leaving a black background

Uses ANSI "default background" color code after closing tests
so any non-black terminals don't remain polluted.

Fixes #1649
Closes #1912
Use correct github url to find commands.json

Once this is merged:
  - merge the latest changes to commands.json in antirez/redis-doc
  - re-run: utils/generate-command-help.rb > src/help.h

Then we'll have nice and easy tab-completed help in redis-cli again.

Closes #1909