When writing a fake dnsConfig to conf.dnsConfig, set lastChecked to an
hour into the future.  This causes dnsclient_unix.go's
tryUpdate("/etc/resolv.conf") calls to short-circuit and ignore that
/etc/resolv.conf's mtime differs from the test's fake resolv.conf
file.  We only need to zero out lastChecked in teardown.

While here, this makes two other tryUpdate(conf.path) test calls
pointless, since they'll now short circuit too.

Fixes #14437.

Change-Id: Ieb520388e319b9826dfa49f134907f4927608a53
Reviewed-on: https://go-review.googlesource.com/19777
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>