Changes

jobs/bodhi-trigger: read SRPM list from `bodhi-testing.yaml`

Add a new `bodhi-testing.yaml` file containing the list of SRPMs to
test. We'll augment the schema of this file later on to allow e.g.
running only a subset of tests.

This will also in the short-term serve as the source of truth for which
packages are gated on these tests or not. Long-term, we may use this
file slightly differently, though likely the tests to run per component
will still live there (including the default tests to run on any
unlisted component).

While we're here, add a few more core packages to the list.

jobs/test-override: resolve Bodhi IDs to Koji IDs early on

It's silly to do this resolution on each builder separately. Let's just
do it once. That also simplifies the downloading loop.

bodhi-testing.yaml: support specifying tests to run

For some packages, it's silly to run the full testsuite when they're not
involved or tested at all in e.g. the testiso scenarios.

Let's allow configuring the list of tests to run per SRPM.

This is also prep for being able to run e.g. just `basic` for a much
wider set of packages.

README.md: update Jenkins URL

We forgot to update this when we migrated from CentOS infra to Fedora.