tests/test-stbt-run.sh: Simplify tests

There's no reason for these tests to run "press" or "wait_for_match" --
all it does is increase the test runtime. And we can use "assert"
directly from python, instead of writing to a log and later grepping
that log.
test_extra_arguments: Test "--" argument to stbt-run

Commit bf393c1d allowed users to pass arguments to the test script on
the stbt-run command line. You can put a "--" between the normal
stbt-run flags and these extra arguments, and the "--" isn't passed on
to the user script. This wasn't documented in bf393c1d, but it's just
the behaviour of python's argparse. This test documents that behaviour,
which is relied on by the next commit.
runner/run: Allow test arguments starting with "-"

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.