runner/run: Don't process tests arguments

Although `stbt run` allows us to specify test arguments, if these start
with a dash, these will be processed as `stbt run` arguments instead of
test ones, i.e.:

    stbt run mytest.py --my-flag

In order to get that working we need to use bash's double dash to force it
to stop taking arguments:

    stbt run mytest.py -- --my-flag

Because of this, when using the runner, it's not possible to do:

    run mytest.py --my-flag

And `run mytest.py -- --my-flag` wouldn't be acceptable because the double
dash is used to separate one test and its arguments from another.