Turn `git serve` into a test helper

Turn `git serve` into a test helper

The `git serve` built-in was introduced in ed10cb952d31 (serve:
introduce git-serve, 2018-03-15) as a backend to serve Git protocol v2,
probably originally intended to be spawned by `git upload-pack`.

However, in the version that the protocol v2 patches made it into core
Git, `git upload-pack` calls the `serve()` function directly instead of
spawning `git serve`; The only reason in life for `git serve` to survive
as a built-in command is to provide a way to test the protocol v2
functionality.

Meaning that it does not even have to be a built-in that is installed
with end-user facing Git installations, but it can be a test helper
instead.

Let's make it so.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>