While snap-server is only needed for the optional monitoring daemon,
some tests, notably those testing these optional features, still depend
on it. So, if snap-server is missing, the Haskell tests should not be
run, as they cannot even be build.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>