net/smtp: make test certificate FIPS 140-3 compliant

net/smtp: make test certificate FIPS 140-3 compliant

The key was too small and was getting rejected by crypto/tls if running
tests in FIPS 140-3 mode.

Change-Id: I6a6a4656374b942aeeca55d5c0464c965db0f6de
Reviewed-on: https://go-review.googlesource.com/c/go/+/658935
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Auto-Submit: Filippo Valsorda <filippo@golang.org>