Add test to check all symbols can be loaded

Summary:
make sure all symboles can be loaded to avoid issues like missing
methods in descendants of abstract base class.

Test Plan:
ran it and verified it passes; remove a method in a descendant class
and verified that the test failed.

Reviewers: epriestley, nh

Reviewed By: nh

CC: aran, nh, jungejason

Differential Revision: 1023