Make the extension's test suite less dependent on individual setups

[vinistock]

Currently, the extension's test suite makes several assumptions about what is going to be available in the user's machine. For example, the Shadowenv test assumes that it will be available and the chruby test assumes rubies will be available in certain locations.

This is not friendly to external contributors who may not have their environments configured similarly to our team. Let's ensure that the test suite skips tests that cannot be executed. Let's also try to ensure that we can run as many tests as possible locally.