unit_tests: add basic cap testing functionality

This introduces a basic couple of tests to test the base cap functionality. The idea is to test for this in a multi threaded way to ensure that we provide the consistency guarantees and we don't break anything when we change the underlying hashmaps etc.

We currently still create an instance of XrdMgmOfs, with some of the services set to 0, a TODO item is mocking this as much as possible. The common XrdMgmOfs is shared across test suites so as to prevent costly construction/destruction per test suite

Signed-off-by: Abhishek Lekshmanan abhishek.lekshmanan@cern.ch