math: use constant rather than variable for exported test threshold

This is a minor follow-up on https://golang.org/cl/153059.

TBR=iant

Updates #6794.

Change-Id: I03657dafc572959d46a03f86bbeb280825bc969d
Reviewed-on: https://go-review.googlesource.com/c/153845Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/export_test.go

@@ -9,5 +9,6 @@ var ExpGo = exp
 var Exp2Go = exp2
 var HypotGo = hypot
 var SqrtGo = sqrt
-var ReduceThreshold = reduceThreshold
 var TrigReduce = trigReduce
+
+const ReduceThreshold = reduceThreshold