Server.ServeTLS wraps Server.Serve with added TLS support. This is
particularly useful for serving on manually initialized listeners.
Example use-case includes ability to serve with TLS on listener
provided by systemd's socket activation.

A matching test heavily based on TestAutomaticHTTP2_ListenAndServe
is also included.

Original code by Gurpartap Singh as
https://go-review.googlesource.com/c/38114/

Fixes #13228

Change-Id: I73bb703f501574a84d261c2d7b9243a89fa52d62
Reviewed-on: https://go-review.googlesource.com/44074
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>