acme: prevent discovery network roundtrips in tests
acme: prevent discovery network roundtrips in tests

Client's FetchCert now requires to do the discovery before
fetching a cert from RFC 8555 compliant CA using POST-as-GET
requests.

Some tests initialized the client without providing a directory
metadata, making it do network roundtrips to the default CA.

This CL sets a non-nil directory metadata to prevent unnecessary
network roundtrips to an actual CA during tests.

Change-Id: Icaa6c2c2fd25ae8e5b1b44c6c3dafa246e6bdbb0
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/200637
Run-TryBot: Alex Vaghin <30120f6dd998b42cadd5816c0710cb48b67e1d08@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/crypto,golang/crypto,golang/crypto