Address github comments

tools/profiling/microbenchmarks/bm_diff/bm_speedup.py

@@ -17,6 +17,7 @@
 from scipy import stats
 import math
 
+_DEFAULT_THRESHOLD = 1e-10
 
 def scale(a, mul):
   return [x * mul for x in a]
@@ -26,7 +27,7 @@ def cmp(a, b):
   return stats.ttest_ind(a, b)
 
 
-def speedup(new, old, threshold):
+def speedup(new, old, threshold = _DEFAULT_THRESHOLD):
   if (len(set(new))) == 1 and new == old: return 0
   s0, p0 = cmp(new, old)
   if math.isnan(p0): return 0