[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
diff --git a/clang/bindings/python/tests/cindex/test_linkage.py b/clang/bindings/python/tests/cindex/test_linkage.py
index 6b482f8..cdd97fc 100644
--- a/clang/bindings/python/tests/cindex/test_linkage.py
+++ b/clang/bindings/python/tests/cindex/test_linkage.py
@@ -1,3 +1,8 @@
+import os
+from clang.cindex import Config
+if 'CLANG_LIBRARY_PATH' in os.environ:
+    Config.set_library_path(os.environ['CLANG_LIBRARY_PATH'])
+
 from clang.cindex import LinkageKind
 from clang.cindex import Cursor
 from clang.cindex import TranslationUnit