[libc] Add a new rule add_integration_test.

All existing loader tests are switched to an integration test added with
the new rule. Also, the getenv test is now enabled as an integration test.

All loader tests have been moved to test/integration. Also, the simple
checker library for the previous loader tests has been moved to a
separate directory of its own.

A follow up change will perform more cleanup of the loader CMake rules
to eliminate now redundent options.

Reviewed By: lntue, michaelrj

Differential Revision: https://reviews.llvm.org/D122266