net: stmmac: selftests: Add a test for TBS feature

Add a new test for TBS feature which is used in ETF scheduler. In this
test, we send a packet with a launch time specified as now + 500ms and
check if the packet was transmitted on that time frame.

Changes from v2:
- Use the TBS bitfield
- Remove debug message

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>