tb: pmd: Ensure signal_detect starts low

To make sure that pmd.signal_status comes up at the right time, keep
signal_detect low at the start of simulation. We don't need to set
pmd.rx, because X is the default value (or rather Z is, but it's the
same for our purposes).

Fixes: 1d65661 ("Add pmd")
Signed-off-by: Sean Anderson <seanga2@gmail.com>

tb/pmd.py

@@ -8,7 +8,8 @@ from cocotb.triggers import RisingEdge, Timer
 
 @cocotb.test(timeout_time=100, timeout_unit='us')
 async def test_rx(pmd):
-    pmd.rx.value = BinaryValue('X')
+    pmd.signal_detect.value = 0
+    await Timer(1)
     await cocotb.start(Clock(pmd.rx_clk_125, 8, units='ns').start())
     # random phase
     await Timer(random.randrange(0, 8000), units='ps')