selftests: drivers: hw: Include tc_common.sh in hw_stats_l3

The tests use the constant TC_HIT_TIMEOUT when waiting on the counter
values. However it does not include tc_common.sh where the counter is
specified. The test has been robust in our testing, which means the counter
is bumped quickly enough that the updated value is available already on the
first iteration. Nevertheless it's not correct. Include tc_common.sh as
appropriate.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>