net/smtp: set ServerName in StartTLS, as now required by crypto/tls

net/smtp: set ServerName in StartTLS, as now required by crypto/tls

the crypto/tls revision d3d43f270632 (CL 67010043, requiring ServerName or InsecureSkipVerify) breaks net/smtp,
since it seems impossible to do SMTP via TLS anymore. i've tried to fix this by simply using a tls.Config with
ServerName, instead of a nil *tls.Config. without this fix, doing SMTP with TLS results in error "tls: either
ServerName or InsecureSkipVerify must be specified in the tls.Config".

testing: the new method TestTlsClient(...) sets up a skeletal smtp server with tls capability, and test client
injects a "fake" certificate allowing tls to work on localhost; thus, the modification to SendMail(...) enabling
this.

Fixes #7437.

LGTM=bradfitz
R=golang-codereviews, josharian, bradfitz
CC=golang-codereviews
https://golang.org/cl/70380043