I had to read documentation to figure out how to hide the test panel. I'm a brand new user to Sublime, so maybe that's a convention in the application, but it'd be more intuitive if the Command-Shift-X ("Show test panel") command would also hide the test panel if it's open.
If this isn't possible within Sublime, perhaps a separate "Hide test panel" command?
I'd be happy to work on a pull request for this if someone could guide the way.
by default 'esc' closes the test panel