GVFS 1.15.4 Fixes MTP and FUSE Issues

GVFS 1.15.4 fixes a crash when libmtp operations failed without an error message, remove_source is now respected when pulling a file and in do_push on MTP support, installs the systemd tmpfiles.d exclusion file for FUSE filesystems, makes use of the new ID_FS_BOOT_SYSTEM_ID udev property for gdu and udisks2. GVFS 1.15.4 also adds gvfs-testbed to enable tests that need root, and checks for URI string presence on CLI (Command-Line Interface) unmount.