Change run.py to use absolute LD_LIBRARY_PATH for --asan

Keith Smith reported this pain point for using the Python --asan option. 

Running from build_posix and using LD_LIBRARY_PATH=.libs is the usual prerequisite for running Python tests.  However, this doesn't work with the --asan options: ASAN stack traces aren't annotated correctly for libraries that would be found using relative paths.  One can manually change LD_LIBRARY_PATH (if you know about this issue), but run.py could also rewrite all parts of LD_LIBRARY_PATH to use absolute paths.