cmake (Windows): let the `.dll` files be found when running the tests

cmake (Windows): let the `.dll` files be found when running the tests

Contrary to Unix-ish platforms, the dependencies' shared libraries are
not usually found in one central place. In our case, since we use
`vcpkg`, they are to be found inside the `compat/vcbuild/vcpkg/` tree.

Let's make sure that they are in the search path when running the tests.

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