tests/test-stbt-batch.sh: Increase timeouts

Self-tests that are testing that `stbt batch run` (without `-1`) keeps
on running until one of the test scripts fails, have a timeout to avoid
the self-test running forever in the (hypothetical) presence of a bug
in `stbt batch run`. This timeout needs to be increased for systems that
are running the self-tests while under heavy load. The increased timeout
shouldn't make any difference to the run-time of these self-tests in the
normal (successful) case.

Thanks to Lewis Haley for reporting the problem:

> I got false negatives for these tests because they were timing out
> before they could conclude. This was probably due to my system having
> a heavy load at the time (running tests with extra logging processes)
> and the stb-tester selftests in parallel.

I also did some related cleanup: I added the `-1` argument to a
self-test that isn't explicitly testing the exit-on-failure behaviour,
but was relying on the fact that `tests/test.py` returns failure the
second time it is run. And I added a timeout to all remaining self-tests
that call `stbt batch run` without `-1`, for consistency.