qa/suites/rbd: disable workunit timeout for dynamic_features_no_cache

qa/suites/rbd: disable workunit timeout for dynamic_features_no_cache

The I/O workload in this test is xfstests (qa/run_xfstests_qemu.sh)
which isn't subjected to any timeout other than global max_job_time
limit in any other subsuite (e.g. qemu/workloads/qemu_xfstests.yaml).
But here, there is a parallel "op" workload defined as a workunit.
The workunit task has a default timeout of 3 hours which is effectively
imposed on the entire job.  In the "rbd cache = false" configuration,
it's sometimes exceeded.

Fixes: https://tracker.ceph.com/issues/48038
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>