oauth: Add unit tests for multiplexer handling

oauth: Add unit tests for multiplexer handling

To better record the internal behaviors of oauth-curl.c, add a unit test
suite for the socket and timer handling code. This is all based on TAP
and driven by our existing Test::More infrastructure.

This commit is a replay of 1443b6c0e, which was reverted due to
buildfarm failures. Compared with that, this version protects the build
targets in the Makefile with a with_libcurl conditional, and it tweaks
the code style in 001_oauth.pl.

Reviewed-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CAOYmi+nDZxJHaWj9_jRSyf8uMToCADAmOfJEggsKW-kY7aUwHA@mail.gmail.com
Discussion: https://postgr.es/m/CAOYmi+m=xY0P_uAzAP_884uF-GhQ3wrineGwc9AEnb6fYxVqVQ@mail.gmail.com