Pass the worker to the build factory

[encukou]

We used to pluck the following attributes from CPythonWorker, our "dataclass-ish" config object, and pass them to builds:

• parallel_tests (via lookup in parallel, a global dict)
• timeout_factor (via extra_factory_args)
• exclude_test_resources (via extra_factory_args)

Instead, pass in the whole CPythonWorker.

Also: Simplify if/else logic for adding the -j option, with a helper that can return an empty list.

Also: Remove "-j2" from class-level testFlags: it always matched the default or was overridden.

[encukou]

@zware, this is where I got from dfac4f3. It doesn't rename parallel/parallel_tests to parallel_processes (yet). A rename would conflict with worker config changes, making things e.g. harder to revert, so it's better to do as a separate PR after some time (if at all).

[zware]

LGTM :)