Revert change to mirror threshold

Reverts part of kubernetes-sigs#3508

See kubernetes-sigs#3708; Envoy
impl cannot meet this fine grain threshold.

Author: howardjohn

conformance/tests/httproute-request-percentage-mirror.go

@@ -39,7 +39,7 @@ import (
 
 const (
 	concurrentRequests    = 10
-	tolerancePercentage   = 5.0
+	tolerancePercentage   = 15.0
 	totalRequests         = 500.0
 	numDistributionChecks = 5
 )