net: selftests: add PHY-loopback test for bad TCP checksums

net: selftests: add PHY-loopback test for bad TCP checksums

Detect NICs and drivers that either drop frames with a corrupted TCP
checksum or, worse, pass them up as valid.  The test flips one bit in
the checksum, transmits the packet in internal loopback, and fails when
the driver reports CHECKSUM_UNNECESSARY.

Discussed at:
https://lore.kernel.org/all/20250625132117.1b3264e8@kernel.org/

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250717083524.1645069-1-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>