kbuild: skip building kselftest when not required by jobfilter (speed up bisection)

[nuclearcat]

Goal

To speed up bisection, do not build kselftest when it isn't needed by the requested jobs (kselftest builds are expensive and dominate bisection turnaround).

This was originally attempted in #2729, but that approach needs more design work before it can land — see below. Converting to an issue to track the proper solution.

Current attempt (#2729)

In kernelci/kbuild.py, when a jobfilter is present, kselftest is disabled unless <build-name>-kselftest literally appears in the filter:

if node['jobfilter'] and self._kfselftest is True:
    kselftest_name = node['name'] + "-kselftest"
    if kselftest_name not in node['jobfilter']:
        self._kfselftest = False

Why this isn't enough

1. Downstream tests are invisible to the build. kbuild.py runs in the build container and only receives node + params + a flat jobfilter list of strings. It has no knowledge of which tests consume kselftest artifacts. So a jobfilter containing a kselftest test (but not the kselftest build name) would silently produce a build without kselftest, and the test would fail mysteriously. The dependency knowledge lives in kernelci-pipeline:

   • test → needs-kselftest is identifiable from config/jobs.yaml (test_method: kselftest, kcidb_test_suite: kselftest.*)
   • build → test link is the event field in config/scheduler.yaml

2. Possible naming bug. Dedicated kselftest build variants are already named kbuild-...-kselftest (e.g. config/jobs.yaml, scheduler.yaml). For such a node, node['name'] + "-kselftest" becomes kbuild-...-kselftest-kselftest, which never matches the filter — so kselftest would be disabled on the very build meant to produce it (this is exactly the bisection-of-a-kselftest-test path). Needs verification against real node['name'] values.

Proposed direction

Move the decision to kernelci-pipeline, which already has self._configs loaded and builds the jobfilter in get_jobfilter() (src/lava_callback.py):

• When any requested job in the jobfilter resolves to a kselftest test (test_method == 'kselftest'), auto-enable kselftest on the corresponding build (e.g. set a kselftest: enable param on the build node, or ensure the kselftest-enabled build name is in the filter).
• Otherwise, leave kselftest off so bisection builds stay fast.

The one non-trivial piece is reverse-resolving the scheduler graph (test → triggering build-node event → build job), since the build job doesn't inherently know its downstream tests. This is an in-memory config walk, not new infrastructure.

Closes #2729 (superseded by this issue).