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_base.py

@@ -1,7 +1,11 @@
 # Checks that an instance type inheriting from a native base that uses
 # MP_TYPE_FLAG_ITER_IS_STREAM will still have a getiter.
 
-import io
+try:
+    import io
+except ImportError:
+    print("SKIP")
+    raise SystemExit
 
 a = io.StringIO()
 a.write("hello\nworld\nmicro\npython\n")