drd/Testing.txt: Document how to run all regression tests with conflict set verification enabled

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14248 a5019735-40e9-0310-863c-91ae7b9d1cf9
drd: Move VG_(getenv)("DRD_VERIFY_CONFLICT_SET")

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14249 a5019735-40e9-0310-863c-91ae7b9d1cf9