selftests: drv-net: add HW timestamping tests

selftests: drv-net: add HW timestamping tests

Add simple tests to validate that the driver sets up timestamping
configuration according to what is reported in capabilities.

For RX timestamping we allow driver to fallback to wider scope for
timestamping if filter is applied. That actually means that driver
can enable ptpv2-event when it reports ptpv2-l4-event is supported,
but not vice versa.

Signed-off-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://patch.msgid.link/20250923173310.139623-5-vadim.fedorenko@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>