test-tool run-command: learn to run (parts of) the testsuite

test-tool run-command: learn to run (parts of) the testsuite

Git for Windows jumps through hoops to provide a development environment
that allows to build Git and to run its test suite. To that end, an
entire MSYS2 system, including GNU make and GCC is offered as "the Git
for Windows SDK". It does come at a price: an initial download of said
SDK weighs in with several hundreds of megabytes, and the unpacked SDK
occupies ~2GB of disk space.

A much more native development environment on Windows is Visual Studio.
To help contributors use that environment, we already have a Makefile
target `vcxproj` that generates a commit with project files (and other
generated files), and Git for Windows' `vs/master` branch is
continuously re-generated using that target.

The idea is to allow building Git in Visual Studio, and to run
individual tests using a Portable Git.

The one missing thing is a way to run the entire test suite: neither
`make` nor `prove` are required to run Git, therefore Git for Windows
does not support those commands in the Portable Git.

To help with that, add a simple test helper that exercises the
`run_processes_parallel()` function to allow for running test scripts in
parallel (which is really necessary, especially on Windows, as Git's
test suite takes such a long time to run).

This will also come in handy for the upcoming change to our Azure
Pipeline: we will use this helper in a Portable Git to test the Visual
Studio build of Git.

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