Add toolchain serverspec

What does this MR do?

⚠ commits are individually reviewable.

Uses the Serverspec ruby gem to validate image contents. This MR adds the toolchain role to the list of tested roles (previously only base)

For each dependency, we test that it is installed, but also that it can perform some basic function (for example, pipenv can install a Pipfile, fastlane allows us to work with an xcode project, ...)

A number of bugs in our images were found during this implementation, they are getting fixed in !91 (merged)

Why was this MR needed?

We need to check that the images we're release are actually working, in an automated fashion

What's the best way to test this MR?

scripts/dev-vm --base-image toolchain-11.img new
scripts/dev-vm --base-image toolchain-11.img --role toolchain spec

# specs should pass!

What are the relevant issue numbers?

See #39 (closed)