run-tests.py: add TESTDIR to PATH if it differs from RUNTESTDIR Before this patch, `RUNTESTDIR` is added to `PATH`, but `TESTDIR` isn't. This doesn't cause any problems, if `run-tests.py` runs in `tests` directory of Mercurial source tree. In this case, `RUNTESTDIR` should be equal to `TESTDIR`. On the other hand, if `run-tests.py` runs in `tests` of third party tools, commands in that directory should be executed with explicit `$TESTDIR/` prefix in `*.t` test scripts. This isn't suitable for the policy "drop explicit $TESTDIR from executables" of Mercurial itself (see 4d2b9b304ad0). BTW, 4d2b9b304ad0 describes that "$TESTDIR is added to the path" even though `TESTDIR` isn't added to `PATH` exactly speaking, because `TESTDIR` and `RUNTESTDIR` weren't yet distinguished from each other at that time. This is a one of preparations for issue4677.