selftests/tc-testing: Add a test for HFSC eltree double add with reentrant enqueue...

selftests/tc-testing: Add a test for HFSC eltree double add with reentrant enqueue behaviour on netem

Reproduce the UAF scenario where netem is a child of HFSC and HFSC
is configured to use the eltree. In such case, this TDC test would
cause the HFSC class to be added to the eltree twice resulting
in a UAF.

Reviewed-by: Victor Nogueira <victor@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Link: https://patch.msgid.link/20250522181448.1439717-3-pctammela@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>