tests/uselect_poll_basic: Add basic test for uselect.poll invariants.

This test doesn't check the actual I/O behavior, just "static" invariants
like behavior on duplicate calls or calls when I/O object is not registered
with poller.

tests/extmod/uselect_poll_basic.py

+try:
+    import usocket as socket, uselect as select, uerrno as errno
+except ImportError:
+    try:
+        import socket, select, errno
+    except ImportError:
+        print("SKIP")
+        raise SystemExit
+
+
+poller = select.poll()
+
+s = socket.socket()
+
+poller.register(s)
+# https://docs.python.org/3/library/select.html#select.poll.register
+# "Registering a file descriptor that’s already registered is not an error,
+# and has the same effect as registering the descriptor exactly once."
+poller.register(s)
+
+# 2 args are mandatory unlike register()
+try:
+    poller.modify(s)
+except TypeError:
+    print("modify:TypeError")
+
+poller.modify(s, select.POLLIN)
+
+poller.unregister(s)
+
+try:
+    poller.modify(s, select.POLLIN)
+except OSError as e:
+    assert e.args[0] == errno.ENOENT