Fix tests with static build on Windows

This merge request is the logical next step after !2438 (merged) and only makes sense after merging the latter merge request.

As GLib is using the DLL files paths to find external helper executables, it cannot work anymore in static build as all relative paths are searched relatively to the running executable file (which can be located wherever).

This patch proposes a straightforward approach to look for the corresponding external helper executable and fixes other tests that were broken by the static build implementation.

Once merged, all tests working (or failing) with dynamic and static builds on Windows will work the same.