selftests: drv-net: xdp: Extract common XDP_TX setup/validation.

selftests: drv-net: xdp: Extract common XDP_TX setup/validation.

In preparation of single-buffer XDP_TX tests, refactor common test code
into the _test_xdp_native_tx method. Add support for multiple payload
sizes, and additional validation for RX packet count. Pass the -n flag
to echo to avoid adding an extra byte into the TX packet.

Signed-off-by: Dimitri Daskalakis <dimitri.daskalakis1@gmail.com>
Link: https://patch.msgid.link/20250821014023.1481662-2-dimitri.daskalakis1@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>