test/src: fixes to Makefile.am for distcheck
test/src: fixes to Makefile.am for distcheck

Calling `make distcheck` simulates running `make check` from a release
tarball.  It also use a separate build directory, e.g.,

    mkdir -p build/_sub
    cd build/_sub
    ../../configure
    make check

This means that we cannot rely on '../..' to be the top source directory
of the mdnsd project tree.  Instead, we use `$(top_srcdir)`.

Also included, minor restructure of Makefile.am, e.g. alphaptetical
listings, 'TESTS +=', etc.

Signed-off-by: Joachim Wiberg <583c295fd7602c168ad814279bbc3894ba65f5d6@gmail.com>

troglobit/mdnsd,troglobit/mdnsd