Fix failing tests due to floating point imprecision

get_sim_time can return floating point values. This will cause tests to
fail since there is an epsilon of error. Fix this by timing things in
steps (which is always an int).

Fixes: 0495ae3 ("Add TX MAC (most of it)")
Signed-off-by: Sean Anderson <seanga2@gmail.com>

tb/axis_mii_tx.py

@@ -10,7 +10,7 @@ from cocotb.binary import BinaryValue
 from cocotb.clock import Clock
 from cocotb.regression import TestFactory
 from cocotb.triggers import ClockCycles, Edge, FallingEdge, First, RisingEdge, Timer
-from cocotb.utils import get_sim_time
+from cocotb.utils import get_sim_time, get_sim_steps
 
 from . import axis_replay_buffer
 from .pcs_rx import mii_recv_packet
@@ -191,12 +191,12 @@ async def test_send(mac, ratio):
         recv = await cocotb.start(recv_packet(mac))
 
         # Measure the IPG to ensure throughput
-        start = get_sim_time('ns')
+        start = get_sim_time('step')
         while not mac.mii_tx_en.value:
             await RisingEdge(mac.clk)
         # The first IPG may not be exact
         if i:
-            assert get_sim_time('ns') - start == 12 * 80 - 4
+            assert get_sim_time('step') - start == get_sim_steps(12 * 80 - 4, 'ns')
 
         compare(await recv.join(), packet)
         assert await status.join() == Status.OK
@@ -254,9 +254,10 @@ async def test_backoff(mac):
         then = None
         for n in range(collisions):
             await restart(mac, 0)
-            now = get_sim_time('ns')
+            now = get_sim_time('step')
             if then is not None:
-                assert now - then <= (8 + 4 + 12 + 2 ** min(n, 10)) * BYTE_TIME_NS
+                assert now - then <= \
+                    get_sim_steps((8 + 4 + 12 + 2 ** min(n, 10)) * BYTE_TIME_NS, 'ns')
             then = now
 
         if collisions == 16: