tests: Require SSL certificate file to be available for test to run.

Previously, any test needing an SSL certificate file would automatically
skip if the file could not be found.  But that makes it too easy to
accidentally skip tests.

Instead, change it so that the test fails if the certificate file doesn't
exist.  That matches, for example, the fact that the test fails if
networking (LAN, WiFi) is not active.

Signed-off-by: Damien George <damien@micropython.org>

tests/multi_net/asyncio_tls_server_client.py

@@ -14,13 +14,6 @@ PORT = 8000
 cert = cafile = "ec_cert.der"
 key = "ec_key.der"
 
-try:
-    os.stat(cafile)
-    os.stat(key)
-except OSError:
-    print("SKIP")
-    raise SystemExit
-
 
 async def handle_connection(reader, writer):
     data = await reader.read(100)