Fixes a regression introduced in
560cb51e7b, which would lead this test on
x86_64-linux to return a DIFF with two packages, nhc98 and dev86 (both
have #:system "i686-linux" and thus depend on a different glibc object;
why other system-specific packages such as 'wine' aren't reported is
unclear).
* tests/graph.scm ("node-transitive-edges + node-back-edges"): Use
'test-equal'. Define 'system-specific?' and use it.
