[yaml2obj] Enable and fix tests

Summary:
These tests were not being run because the yaml extension
wasn't be picked up by lit.

This change also fixes the tests which themselves were broken.

Patch By: Sam Clegg

Reviewers: beanz

Reviewed By: beanz

Subscribers: fhahn

Differential Revision: https://reviews.llvm.org/D31436

llvm-svn: 299088