tests/net_hosted: Only run network loopback test on supported targets.

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>

tests/net_hosted/asyncio_loopback.py

@@ -1,5 +1,12 @@
 # Test network loopback behaviour
 
+import sys
+
+# Only certain platforms can do TCP/IP loopback.
+if sys.platform not in ("darwin", "esp32", "linux"):
+    print("SKIP")
+    raise SystemExit
+
 try:
     import asyncio
 except ImportError: