ci: also build and test with MS Visual Studio on Azure Pipelines

ci: also build and test with MS Visual Studio on Azure Pipelines

... because we can, now. Technically, we actually build using `MSBuild`,
which is however pretty close to building interactively in Visual
Studio.

As there is no convenient way to run Git's test suite in Visual Studio,
we unpack a Portable Git to run it, using the just-added test helper to
allow running test scripts in parallel.

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