acme: increase pebble test waitForServer attempts

In CI it seems that occasionally we can't connect to the test servers
within 10 tries, and the test flakes. Let's give the process more
attempts.

Updates golang/go#74437

Change-Id: I74d6cea83468a3a572ec4b52ff7314c778c664cf
Cq-Include-Trybots: luci.golang.try:x_crypto-gotip-linux-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/692075
Auto-Submit: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Mark Freeman <mark@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

acme/pebble_test.go

@@ -692,14 +692,14 @@ func startPebbleEnvironment(t *testing.T, config *environmentConfig) environment
 func waitForServer(t *testing.T, addr string) {
 	t.Helper()
 
-	for i := 0; i < 10; i++ {
+	for i := 0; i < 20; i++ {
 		if conn, err := net.Dial("tcp", addr); err == nil {
 			conn.Close()
 			return
 		}
 		time.Sleep(time.Duration(i*100) * time.Millisecond)
 	}
-	t.Fatalf("failed to connect to %q after 10 tries", addr)
+	t.Fatalf("failed to connect to %q after 20 tries", addr)
 }
 
 // fetchModule fetches the module at the given version and returns the directory