selftests/tc-testing: add tests for cls_fw and cls_flow on shared blocks

selftests/tc-testing: add tests for cls_fw and cls_flow on shared blocks

Regression tests for the shared-block NULL derefs fixed in the previous
two patches:

  - fw: attempt to attach an empty fw filter to a shared block and
    verify the configuration is rejected with EINVAL.
  - flow: create a flow filter on a shared block without a baseclass
    and verify the configuration is rejected with EINVAL.

Signed-off-by: Xiang Mei <xmei5@asu.edu>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20260331050217.504278-3-xmei5@asu.edu
Signed-off-by: Paolo Abeni <pabeni@redhat.com>