Refactoring out some pointless sys.path changes
Calling PrepareClangFlags is not needed anymore

ycm_extra_conf.py files used to import clang_helpers and then use the
PrepareClangFlags function; this is now unnecessary since the logic from that
function has been moved to flags.py. The old PrepareClangFlags function is still
there (it just returns the flags it gets) for the sake of backwards
compatibility with old ycm_extra_conf.py files.
Tests for helper functions in flags.py
Running nosetests after install.sh

ycm_core.so needs to be built for the tests to run. The python code imports
ycm_core, so it needs to be present.