acme: capture pebble test subprocess stdout/stderr

When spawning the pebble and pebble-challtestserv processes redirect
stdout/stderr to bytes.Buffer instances and print their content at test
end as appropriate.

The stdout/stderr content for each process is printed if the test
failed, or if testing is being done in verbose mode. Otherwise the
output is swallowed.

This makes debugging test failures much easier as output from the
subprocesses from independent tests isn't intermingled.

Updates golang/go#74437

Cq-Include-Trybots: luci.golang.try:x_crypto-gotip-linux-amd64-longtest
Change-Id: Ia79a3609ce3522ef6248442de247554c39367162
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/686935
Auto-Submit: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

acme/pebble_test.go

@@ -776,14 +776,28 @@ func prepareBinaries(t *testing.T, pebbleDir string) string {
 func spawnServerProcess(t *testing.T, dir string, cmd string, args ...string) {
 	t.Helper()
 
+	var stdout, stderr bytes.Buffer
+
 	cmdInstance := exec.Command("./"+cmd, args...)
 	cmdInstance.Dir = dir
-	cmdInstance.Stdout = os.Stdout
-	cmdInstance.Stderr = os.Stderr
+	cmdInstance.Stdout = &stdout
+	cmdInstance.Stderr = &stderr
+
 	if err := cmdInstance.Start(); err != nil {
 		t.Fatalf("failed to start %s: %v", cmd, err)
 	}
+
 	t.Cleanup(func() {
 		cmdInstance.Process.Kill()
+
+		if t.Failed() || testing.Verbose() {
+			t.Logf("=== %s output ===", cmd)
+			if stdout.Len() > 0 {
+				t.Logf("stdout:\n%s", strings.TrimSpace(stdout.String()))
+			}
+			if stderr.Len() > 0 {
+				t.Logf("stderr:\n%s", strings.TrimSpace(stderr.String()))
+			}
+		}
 	})
 }