selftests: forwarding: sch_tbf_*: Add a pre-run hook

The driver-specific wrappers of these selftests invoke bail_on_lldpad to
make sure that LLDPAD doesn't trample the configuration. The function
bail_on_lldpad is going to move to lib.sh in the next patch. With that, it
won't be visible for the wrappers before sourcing the framework script. And
after sourcing it, it is too late: the selftest will have run by then.

One option might be to source NUM_NETIFS=0 lib.sh from the wrapper, but
even if that worked (it might, it might not), that seems cumbersome. lib.sh
is doing fair amount of stuff, and even if it works today, it does not look
particularly solid as a solution.

Instead, introduce a hook, sch_tbf_pre_hook(), that when available, gets
invoked. Move the bail to the hook.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>