all: fix "result not used" vet warnings

For tests, assign to _.
For benchmarks, assign to a sink.

Updates #11041

Change-Id: I87c5543245c7bc74dceb38902f4551768dd37948
Reviewed-on: https://go-review.googlesource.com/27116
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/net/ip_test.go b/src/net/ip_test.go
index b6ac26d..0ef46ee 100644
--- a/src/net/ip_test.go
+++ b/src/net/ip_test.go
@@ -242,13 +242,15 @@
 	}
 }
 
+var sink string
+
 func BenchmarkIPString(b *testing.B) {
 	testHookUninstaller.Do(uninstallTestHooks)
 
 	for i := 0; i < b.N; i++ {
 		for _, tt := range ipStringTests {
 			if tt.in != nil {
-				tt.in.String()
+				sink = tt.in.String()
 			}
 		}
 	}
@@ -299,7 +301,7 @@
 
 	for i := 0; i < b.N; i++ {
 		for _, tt := range ipMaskStringTests {
-			tt.in.String()
+			sink = tt.in.String()
 		}
 	}
 }