uasyncio.core: Add test for full wait for a time-scheduled coro.

Specifically, that a coroutine scheduled to run at some time (after some
delay) waits requested time before it's run and not run prematurely in
case an I/O completion happens before it.

uasyncio.core/test_full_wait.py

@@ -0,0 +1,34 @@
+# Test that coros scheduled to run at some time don't run prematurely
+# in case of I/O completion before that.
+import uasyncio.core as uasyncio
+
+class MockEventLoop(uasyncio.EventLoop):
+
+    def __init__(self):
+        super().__init__()
+        self.t = 0
+        self.msgs = []
+
+    def time(self):
+        return self.t
+
+    def pass_time(self, delta):
+        self.t += delta
+
+    def wait(self, delay):
+        self.pass_time(100)
+        if self.t == 100:
+            self.call_soon(lambda: self.msgs.append("I should be run first, time: %s" % self.time()))
+        if self.t == 1000:
+            raise StopIteration
+
+
+loop = MockEventLoop()
+loop.call_later_ms_(500, lambda: loop.msgs.append("I should be run second, time: %s" % loop.time()))
+try:
+    loop.run_forever()
+except StopIteration:
+    pass
+
+print(loop.msgs)
+assert loop.msgs == ['I should be run first, time: 100', 'I should be run second, time: 500']