[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