Fix race condition in dff_cocotb test

There is no constraint on tb.stop() action with regards to all other
coroutines waiting for clock rising edge. This may lead to mismatching
transactions and expectations being sent to Scoreboard and the following
error:

AssertionError: Received a transaction but wasn't expecting anything

Waiting on ReadOnly trigger ensures that all other coroutines have
performed their actions before the testbetch is stopped.

examples/dff/tests/dff_cocotb.py

@@ -30,7 +30,7 @@
 
 import cocotb
 from cocotb.clock import Clock
-from cocotb.triggers import RisingEdge
+from cocotb.triggers import ReadOnly, RisingEdge
 from cocotb.binary import BinaryValue
 from cocotb.regression import TestFactory
 
@@ -149,6 +149,8 @@ async def run_test(dut):
 
     # Stop generation of input data. One more clock cycle is needed to capture
     # the resulting output of the DUT.
+
+    await ReadOnly()
     tb.stop()
     await clkedge