[python] [tests] Support overriding library path via environment

Support a new CLANG_LIBRARY_PATH environment variable for the Python
binding tests.  This variable can be used to force the bindings to load
libclang.* from a specific directory.

I plan to use this when integrating Python binding tests with the CMake
build system.  Currently, those tests load libclang.so from default
search paths, so I would have to rely on platform-specific mechanics
such as LD_LIBRARY_PATH.  Instead of copying the whole logic necessary
to handle platform differences into yet another place, it's easier to
just add a dedicated variable for this purpose.

Differential Revision: https://reviews.llvm.org/D52806

llvm-svn: 344240