net/smtp: fix dropped test error
net/smtp: fix dropped test error

Pick up a dropped error in TestSendMailWithAuth() and simplify goroutine
to use an error channel instead of a sync.WaitGroup and an empty struct
doneCh.

Change-Id: Ie70d0f7c4c85835eb682e81d086ce4d9900269e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/205247
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

CAFxX/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,akutz/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,akutz/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,golang/go,CAFxX/go,akutz/go,akutz/go,golang/go,golang/go,cloudflare/go,akutz/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go