tests/basics: Skip tests of io module individually using SKIP.

Instead of using a feature check.  This is more consistent with how other
optional modules are skipped.

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

tests/basics/io_stringio_with.py

@@ -1,4 +1,9 @@
-import io
+try:
+    import io
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
 # test __enter__/__exit__
 with io.StringIO() as b:
     b.write("foo")