Add documentation on how to use the testbed.

[MScottAdams]

The testbed

1. Testbed requires GIT to be installed. (I don't use GIT I use SVN). Installing GIT allowed the testbed to load and work in Unity 2021.1.26f1

2. Once it was running I didn't know how to use it. I dug into the code and finally found it was waiting on space_bar to step through the different tests. Some tests had buttons that can be pressed.

Everything worked after that.

[rlabrecque]

Do you have any insight into why Git needs to be installed? Do you need Git installed for Unity's package manager 👀?