Adds initial TDL test cases

[popematt]

Issue #, if available:

#88

Description of changes:

• Adds a directory for TDL test cases. This specifically tests things in a template body—not any other part of the macro definition.
• Adds test cases for if_* special forms and expression groups
• Adds test files with description of test cases to write for all other aspects of TDL.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.