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>