fixed bad logic introducted to test_observe

`test_observe_value_times_out_with_busy_sleep` should be using a
blocking thread sleep instead of an async one.

tests/core/test_observe.py

@@ -86,7 +86,10 @@ async def tick():
 
     async def watch():
         async for val in observe_value(sig, done_timeout=0.2):
-            await asyncio.sleep(0.1)
+            # This is a test to prove a subtle timing bug where the inner loop
+            # of observe_value was blocking the event loop.
+            # This gives enough time for two ticks, but no more.
+            time.sleep(0.09)
             recv.append(val)
 
     t = asyncio.create_task(tick())
@@ -95,7 +98,7 @@ async def watch():
         with pytest.raises(asyncio.TimeoutError):
             await watch()
         assert recv == [0, 1]
-        assert time.time() - start == pytest.approx(0.2, abs=0.05)
+        assert time.time() - start == pytest.approx(0.25, abs=0.05)
     finally:
         t.cancel()