Introduce kernel rebuild CI test.

Introduce baseos-qe.koji-build.scratch-build.validation as a new
gating test for binutils.  The test makes sure that the binutils
update under test doesn't break the kernel build process.  The
testing kernel build happens in koji in a side-tag buildroot
containing the binutils build under test.

The source code of this new testcase lives here:
https://github.com/fedora-ci/scratch-build-test

In case of a failure that is considered a false positive, the test
result can be waived using Bodhi this way:
https://docs.fedoraproject.org/en-US/ci/gating/#_waive

gating.yaml

@@ -5,6 +5,7 @@ decision_context: bodhi_update_push_stable
 subject_type: koji_build
 rules:
   - !PassingTestCaseRule {test_case_name: fedora-ci.koji-build.tier0.functional}
+  - !PassingTestCaseRule {test_case_name: baseos-qe.koji-build.scratch-build.validation}
 --- !Policy
 product_versions:
   - rhel-9