Add tests for exported generated headers

Improves the test coverage for exporting of generated headers. These
tests highlight the bug described in b/180712399.

Follow up changes will refactor and fix bugs in this code. Adding the
tests separately help ensure that those changes do not inadvertantly
change the behavior.

Test: m nothing
Bug: 180712399
Change-Id: I0225b0cf53259071edb99a94be5014ed0e019bde