run_tests.py can't find tor via relative pathname if cwd not the top of the stem tree

When running run_tests.py with a current working directory that's not the top of the stem source tree, it can't find a tor binary via a relative pathname. This prevents make test-stem from working in the tor source tree. (run_tests.py seems to mostly work if given a relative pathname to tor when run from the top of the stem source tree.)

tlyu@arcadia:~/src/tor$ ../stem/run_tests.py --tor ./src/or/tor --all --log notice --target RUN_ALL

produces many errors such as

Starting ./src/or/tor...

  failed to start tor: './src/or/tor' doesn't exist

A workaround is to provide an absolute pathname for the tor binary, like

tlyu@arcadia:~/src/tor$ ../stem/run_tests.py --tor `pwd`/src/or/tor --all --log notice --target RUN_ALL

but it would be nice if the relative pathname also worked. (Adapting the make test-stem rule in tor is also a possibility.)