denylist: support being able to denylist a subtest

We currently have a specific sub-test that is failing consistently on certain platforms. It would be nice if we could denylist/snooze just the subtest.

With:

- pattern: podman.base/resources                                                                                                             
  tracker: https://github.com/coreos/fedora-coreos-tracker/issues/966                                                                        
  snooze: 2021-10-15                                                                                                                         
  platforms:                                                                                                                                 
    - aws                                                                                                                                    
    - gcp                                                                                                                                    
    - qemu-unpriv

I currently see:

kola -p qemu-unpriv --build 35.20210924.dev.1 --output-dir tmp/kola run podman.base
⚠️  Skipping kola test pattern "fcos.internet":
  👉 https://github.com/coreos/coreos-assembler/pull/1478
⚠️  Skipping kola test pattern "podman.workflow":
  👉 https://github.com/coreos/coreos-assembler/pull/1478
🕒 Snoozing kola test pattern "podman.base/resources" until Oct 15 2021:
  👉 https://github.com/coreos/fedora-coreos-tracker/issues/966
=== RUN   podman.base
=== RUN   podman.base/info
=== RUN   podman.base/resources
--- PASS: podman.base (28.07s)
    --- PASS: podman.base/info (0.27s)
    --- PASS: podman.base/resources (3.76s)
PASS, output in tmp/kola

Which is not correct.

Start by tracing how the kola denylist and snooze configuration handles the shown pattern podman.base/resources, using the example command as the entry point. Check the existing behavior for whole-test patterns and identify the relevant tests or command output checks. Done means the subtest is skipped or snoozed on the listed platforms rather than executed and reported as passed.