testing: quote -test.v=test2json output when logging it

The -test.v=test2json flag causes the testing package to inject extra
control characters in the output to allow the JSON parser to more
gracefully handle extraneous writes to os.Stdout and/or os.Stderr in
the package under test (see CL 443596). However, it doesn't filter out
those control characters because almost no real-world tests will
output them.

It turns out that testing.TestFlag is one of the rare tests that does
output those control characters, because it tests the
-test.v=test2json flag itself.

Fixes #59181.

Change-Id: I35ca6748afcd3d4333563028817caac946f5e86a
Reviewed-on: https://go-review.googlesource.com/c/go/+/479035
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

src/testing/flag_test.go

@@ -39,7 +39,10 @@ func TestFlag(t *testing.T) {
 			cmd.Env = append(cmd.Environ(), flagTestEnv+"=1")
 			b, err := cmd.CombinedOutput()
 			if len(b) > 0 {
-				t.Logf("%s", b)
+				// When we set -test.v=test2json, we need to escape the ^V control
+				// character used for JSON framing so that the JSON parser doesn't
+				// misinterpret the subprocess output as output from the parent test.
+				t.Logf("%q", b)
 			}
 			if err != nil {
 				t.Error(err)