test/mallocfin: relax test to 80% from 90%.

R=r
CC=golang-dev
https://golang.org/cl/725042
diff --git a/test/mallocfin.go b/test/mallocfin.go
index 918b806..da3253a 100644
--- a/test/mallocfin.go
+++ b/test/mallocfin.go
@@ -54,7 +54,7 @@
 		runtime.GC()
 		runtime.Gosched()
 	}
-	if nfinal < N*9/10 {
+	if nfinal < N*8/10 {
 		panic("not enough finalizing:", nfinal, "/", N)
 	}
 }