Add a test script

[marioortizmanero]

It doesn't have to be too detailed, but it'd be nice to have a test script to make sure the most basic functions always work as intended.

• [X] Test for volUp
• [X] Test for volDown
• [X] Test for changeDevice
• [ ] Test for volSync
• [x] Test for volMute (all three cases)
• [ ] listen behaviour (#9 and #20 should be finished first)
• [ ] More specific edge cases

[marioortizmanero]

I think there are enough tests for now.