tests: Move non-filesystem io tests to basics dir with io_ prefix.

tests/basics/io_stringio_with.py

@@ -0,0 +1,9 @@
+try:
+    import uio as io
+except ImportError:
+    import io
+
+# test __enter__/__exit__
+with io.StringIO() as b:
+    b.write("foo")
+    print(b.getvalue())