tests/run-tests.sh: Set PYTHONPATH for irnetbox tests

Commit 9a4d2086 added a test that does `import irnetbox` but
`irnetbox.py` isn't on the PYTHONPATH. (When `stbt-run` imports
`irnetbox` it always worked because the current file's directory is
searched before PYTHONPATH, and `stbt-run` is in the same directory as
`irnetbox.py`).