Prepend LD_LIBRARY_PATH for python tests

* lang/python/tests/Makefile.am (TESTS_ENVIRONMENT): Prepend path
instead of setting the value.

This fixes the case where tools / libararies are needed for
a working GnuPG system that are pointed to by LD_LIBRARY_PATH.
E.g. GnuPG itself is installed in a custom prefix and PATH /
LD_LIBRARY_PATH is set accordingly.