cocotb: Unexpected behavior with simple dff example - unexpected triggering at time==0

From: https://github.com/cocotb/cocotb/pull/1529#pullrequestreview-387867775

# test_dff.py

import random
import cocotb
from cocotb.clock import Clock
from cocotb.triggers import FallingEdge

@cocotb.test()
async def test_dff_simple(dut):
    """ Test that d propagates to q """

    clock = Clock(dut.clk, 10, units="us")  # Create a 10us period clock on port clk
    cocotb.fork(clock.start())  # Start the clock

    for i in range(10):
        await RisingEdge(dut.clk)  # Synchronize ourselves with the clock
        val = random.randint(0, 1)
        dut.d <= val  # Assign the random value val to the input port d
        await FallingEdge(dut.clk)
        assert dut.q == val, "output q was incorrect on the {}th cycle".format(i)

  6000.00ns ERROR    cocotb.regression                         regression.py:396  in _score_test                     Test Failed: test_dff_simple (result was ValueError)
                                                                                                                     Traceback (most recent call last):
                                                                                                                       File "/examples/dff_simple/test_dff.py", line 20, in test_dff_simple
                                                                                                                         assert dut.q == val, "output was incorrect on the {}th cycle".format(i)
                                                                                                                       File "/cocotb/handle.py", line 409, in __eq__
                                                                                                                         return self.value == other
                                                                                                                       File "/cocotb/binary.py", line 439, in __eq__
                                                                                                                         return self.value == other
                                                                                                                       File "/cocotb/binary.py", line 296, in value
                                                                                                                         return self.integer
                                                                                                                       File "/cocotb/binary.py", line 287, in integer
                                                                                                                         return self._convert_from[self.binaryRepresentation](self._str)
                                                                                                                       File "/cocotb/binary.py", line 193, in _convert_from_unsigned
                                                                                                                         return int(resolve(x), 2)
                                                                                                                       File "/cocotb/binary.py", line 45, in resolve
                                                                                                                         raise ValueError("Unable to resolve to binary >%s<" % string)
                                                                                                                     ValueError: Unable to resolve to binary >z<
  6000.00ns ERROR    cocotb.regression                         regression.py:227  in tear_down                       Failed 1 out of 1 tests (0 skipped)