Add test framework to evaluate a template containing expressions

[anthony-c-martin]

I wanted to be able to use the Deployments code in our tests to evaluate ARM expressions. This allows us to go farther than saying "this looks like the correct expression to pass to ARM" and say "this generates an expression which evaluates to the expected value" in our integration tests:

1. Tests are less brittle to the exact code being generated, and easier to review for impact.
2. We gain better test coverage of the Deployment code, allowing us to account for unexpected edge cases.

This is just a start - it is incomplete and I expect us to improve upon it; I just wanted to lay the groundwork.

Fixes #1874

[majastrz]

This is cool!