"make check" broken with everything but tools disabled

[Markus Armbruster]

Watch this:

    $ mkdir bld-tools
    $ cd bld-tools
    $ ../configure --disable-system --disable-user --enable-tools
    $ make check
    [...]
    make: *** No rule to make target 'tests/qemu-iotests/socket_scm_helper', 
needed by 'check-block'.  Stop.