t/oxr Install libopenxr in a subdirectory

Fix for #335 relative to dlopened library.

Manifest becomes for relative path:

{
    "file_format_version": "1.0.0",
    "runtime": {
        "name": "Monado",
        "library_path": "../../../lib64/monado/libopenxr_monado.so"
    }
}

or, absolute path, install prefix=/tmp/monado:

{
    "file_format_version": "1.0.0",
    "runtime": {
        "name": "Monado",
        "library_path": "/tmp/monado/lib64/monado/libopenxr_monado.so"
    }
}