Tests: check for shlibdeps symbols/shlibs before testing it

OS Elbrus 6.x has totally broken dpkg-shlibdeps; 7.1 has
a working one, but still no symbols/shdibdeps files, so
generated dependencies are also empty. Since this commit,
we're checking if these files exist, and if not, we skip
the CPackComponentsDEB-components-depend2 test.