acme: fix pebble subprocess output data race

Wait for process completion before reading stdout/stderr buffers
to eliminate race between I/O Go routines and test cleanup.

Updates golang/go#74437

Cq-Include-Trybots: luci.golang.try:x_crypto-gotip-linux-amd64-longtest-race
Change-Id: I2e650c04db5be0d7a1e858ce40e25f13ad12223c
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/693596
Auto-Submit: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

acme/pebble_test.go

@@ -789,6 +789,7 @@ func spawnServerProcess(t *testing.T, dir string, cmd string, args ...string) {
 
 	t.Cleanup(func() {
 		cmdInstance.Process.Kill()
+		cmdInstance.Wait()
 
 		if t.Failed() || testing.Verbose() {
 			t.Logf("=== %s output ===", cmd)