Only shared libraries are added to PATH for tests and meson devenv on Windows

On Windows, the directories of the shared libraries that an executable links to are added to PATH when running tests and in meson devenv. The directories of other targets listed in depends or in the test arguments, for example a custom_target(), were also added. This was not intended and could cause the wrong DLL to be found. Now only the directories containing shared_library() targets, and the shared libraries that they depend on, are added.

A project that relied on a custom_target() output directory being in PATH should add that directory to the test environment explicitly.