Draft: test-msys2.sh: Copy built DLLs into each directory with tests

This works around a Meson bug where a system-wide installed copy of GLib can incorrectly get used in preference to the just-built copy (see https://github.com/mesonbuild/meson/issues/12330).

In principle some of these are unnecessary (for example libgio-2.0-0.dll only needs to be in gio/tests/ and girepository/tests/) but it's simpler to copy each DLL to each tests directory.

Fixes: c428d6e6 "ci: Build and tar the platform specific documentation"
Resolves: #3262 (closed)