pgsql: 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(at)ilmari(dot)org>
Reviewed-by: Andrew Dunstan <andrew(at)dunslane(dot)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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4e1e417330d42cb19c7d439cd50eea20f25c7518

Modified Files
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src/interfaces/libpq-oauth/Makefile | 36 +-
src/interfaces/libpq-oauth/meson.build | 35 ++
src/interfaces/libpq-oauth/t/001_oauth.pl | 24 ++
src/interfaces/libpq-oauth/test-oauth-curl.c | 527 +++++++++++++++++++++++++++
4 files changed, 618 insertions(+), 4 deletions(-)