ci: use test-tool as unit test runner on Windows

ci: use test-tool as unit test runner on Windows

Although the previous commit changed t/Makefile to run unit tests
alongside shell tests, the Windows CI still needs a separate unit-tests
step due to how the test sharding works.

We want to avoid using `prove` as a test running on Windows due to
performance issues [1], so use the new test-tool runner instead.

[1] https://lore.kernel.org/git/850ea42c-f103-68d5-896b-9120e2628686@gmx.de/

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>