update xcompat tests to use public test vectors

[mattsb42-aws]

Problem

Initial public test vectors are published now: https://github.com/awslabs/aws-encryption-sdk-test-vectors

The xcompat test suites need to be updated to use these rather than just looking for locally.

Solution

The plan for this is to:

1. Add a git submodule directing to the test vectors repo at the path test/vectors
2. Update xcompat tests to two-level parameterization:
   1. For each zip file found in test/vectors/vectors
   2. Load test suites from manifest in zip file
      • Read data from zip file for each test

[mattsb42-aws]

This will use the test_vector_handlers sub-package.