selftests: tc-testing: add tests for qfq mtu sanity check

QFQ only supports a certain bound of MTU size so make sure
we check for this requirement in the tests.

Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Pedro Tammela <pctammela@mojatatu.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Tested-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>