tb: pmd: Allow a larger range for alignment

If we get a run of 1s (or 0s) at the start of the stream, it may take a
while for us to align. Increase the alignment range. Additionally, from
testing, it appears that negative ranges are never used. Just start at
0.

Signed-off-by: Sean Anderson <seanga2@gmail.com>

tb/pmd.py

@@ -66,7 +66,7 @@ async def test_rx(pmd, delays):
 
     best_corr = -1
     best_off = None
-    for off in range(-7, 8):
+    for off in range(16):
         corr = sum(i == o for i, o in zip(ins[off:], outs))
         if corr > best_corr:
             best_corr = corr