ci(visual-studio): actually run the tests in parallel

ci(visual-studio): actually run the tests in parallel

Originally, the CI/PR builds that build and test using Visual Studio
were implemented imitating `linux-clang`, i.e. still using the
`Makefile`-based build infrastructure.

Later (but still before the patches made their way into git.git's
`master`), however, this was changed to generate Visual Studio project
files and build the binaries using `MSBuild`, as this reflects more
accurately how Visual Studio users would want to build Git (internally,
Visual Studio uses `MSBuild`, or at least something very similar).

During that transition, we needed to implement a new way to run the test
suite in parallel, as Visual Studio users typically will only have a Git
Bash available (which does not ship with `make` nor with support for
`prove`): we simply implemented a new test helper to run the test suite.

This helper even knows how to run the tests in parallel, but due to a
mistake on this developer's part, it was never turned on in the CI/PR
builds. This results in 2x-3x longer run times of the test phase.

Let's use the `--jobs=10` option to fix this.

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