tests: get test name from testing.T (#22941)

There were 2 TODOs about that fix after Golang 1.8 release.
It's here for 3 years already, so now should be the right time.

tests/rlp_test.go

@@ -24,7 +24,7 @@ func TestRLP(t *testing.T) {
 	t.Parallel()
 	tm := new(testMatcher)
 	tm.walk(t, rlpTestDir, func(t *testing.T, name string, test *RLPTest) {
-		if err := tm.checkFailure(t, name, test.Run()); err != nil {
+		if err := tm.checkFailure(t, test.Run()); err != nil {
 			t.Error(err)
 		}
 	})