Only a few ports have TCP/IP loopback enabled in their network stack, and
this test will only pass on those ports. There's not really any good way
to do a feature check for loopback mode without actually running the test
and seeing if it passes/fails, so add an explicit check that the test is
running on a port known to support loopback.
(Enabling loopback on lwIP, eg RPI_PICO_W, costs +568 code and +272 bss and
is a rarely used feature, so not worth unconditionally enabling.)
Signed-off-by: Damien George <damien@micropython.org>
