qa/cephfs: override testing kernel with -k option

Normally, the fs suite overrides the kernel branch whenever the kernel client
is used, according to the matrix of configs. This prevents easily testing a new
kernel with the -k option to teuthology-suite. So, using the base config passed
to the script, detect if an alternate testing kernel is desired and use that
instead.

The scheduler still needs to do some work:

    $ teuthology-suite ... -k master --filter k-testing
    ...
    2024-10-17 19:57:52,051.051 DEBUG:teuthology.suite.merge:base kernel {'branch': 'master', 'kdb': 1, 'sha1': 'ccda9910d8490f4fb067131598e4b2e986faa5a0', 'flavor': 'default'}
    2024-10-17 19:57:52,051.051 DEBUG:teuthology.suite.merge:overriding testing kernel with {'branch': 'master', 'kdb': 1, 'sha1': 'ccda9910d8490f4fb067131598e4b2e986faa5a0', 'flavor': 'default'}

vs.

    $ teuthology-suite ... --filter k-testing
    ...
    2024-10-17 20:04:52,265.265 DEBUG:teuthology.suite.merge:base kernel {'branch': 'distro', 'kdb': 1, 'sha1': 'distro'}

Which will only select jobs using the "testing" kernel.

See-also: ceph/teuthology#2008
Fixes: https://tracker.ceph.com/issues/68603
Signed-off-by: Patrick Donnelly <[email protected]>

qa/cephfs/mount/kclient/overrides/distro/testing/k-testing.yaml

@@ -1,3 +1,12 @@
+teuthology:
+  premerge: |
+    log.debug("base kernel %s", base_config.kernel)
+    local kernel = base_config.kernel
+    if kernel.branch ~= "distro" then
+      log.debug("overriding testing kernel with %s", kernel)
+      yaml_fragment.kernel.client = kernel
+    end
+
 kernel:
   client:
     branch: testing