Added setup/teardown for module API tests.

During the one-time setup of the test cases, we do the following:
1. set LD_LIBRARY_PATH to also point to the src/.libs directory.
   The original LD_LIBRARY_PATH is restored at the end of all tests.
2. dlopen() examplemodule library and retrieve the pointer to
   ModuleBase for the test module.  This pointer is later used to
   reset the Mesos and module API versions during per-test teardown.

During the tear-down after each test, we unload the module to allow
later loads to succeed.

Review: https://reviews.apache.org/r/26855