testing: do not print 'root' benchName during test of package testing

$ cd $GOROOT/src/testing
	$ go test
	root
	root
	root
	root
	PASS
	$

The root prints have been happening since Go 1.14.
There is a test in sub_test.go that calls b.Run directly
with a benchmark named "root", which triggers the print.
Silence them.

Change-Id: I2f0c186f04c6139bc24fab0e91975fcf0a8e80fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/421437
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/testing/benchmark.go

@@ -615,6 +615,11 @@ func (ctx *benchContext) processBench(b *B) {
 	}
 }
 
+// If hideStdoutForTesting is true, Run does not print the benchName.
+// This avoids a spurious print during 'go test' on package testing itself,
+// which invokes b.Run in its own tests (see sub_test.go).
+var hideStdoutForTesting = false
+
 // Run benchmarks f as a subbenchmark with the given name. It reports
 // whether there were any failures.
 //
@@ -670,7 +675,9 @@ func (b *B) Run(name string, f func(b *B)) bool {
 			}
 		})
 
-		fmt.Println(benchName)
+		if !hideStdoutForTesting {
+			fmt.Println(benchName)
+		}
 	}
 
 	if sub.run1() {

src/testing/sub_test.go

@@ -657,6 +657,10 @@ func TestBRun(t *T) {
 			}
 		},
 	}}
+	hideStdoutForTesting = true
+	defer func() {
+		hideStdoutForTesting = false
+	}()
 	for _, tc := range testCases {
 		t.Run(tc.desc, func(t *T) {
 			var ok bool