Fixes a bug introduced in golang.org/cl/100078 where incorrect ACME client was used, causing nil pointer dereference. The change also improves related tests, removing code paths diverging in testing. Fixes golang/go#25581 Change-Id: I8c5531fcc5814a5a64f14911c0ad86c476a76d2f Reviewed-on: https://go-review.googlesource.com/114817 Reviewed-by: Filippo Valsorda <filippo@golang.org>