Run tests for graphviz

Now works with QC-2.8 and doesn't depend on dot to run tests

build-constraints.yaml

@@ -1541,7 +1541,6 @@ skipped-tests:
     - bson
     - cabal-install
     - exceptions
-    - graphviz
     - pandoc
     - testpack
     - vector
@@ -1631,9 +1630,6 @@ expected-test-failures:
     # https://github.com/fpco/stackage/issues/226
     - options
 
-    # Depends on a missing graphviz executable
-    - graphviz
-
     # No AWS creds available
     - aws