engine: resources: Add reversible API and file resource

This adds the first reversible resource (file) and the necessary engine
API hooks to make it all work. This allows a special "reversed" resource
to be added to the subsequent graph in the stream when an earlier
version "disappears". This disappearance can happen if it was previously
in an if statement that then becomes false.

It might be wise to combine the use of this meta parameter with the use
of the `realize` meta parameter to ensure that your reversed resource
actually runs at least once, if there's a chance that it might be gone
for a while.

This patch also adds a new test harness for testing resources. It
doesn't test the "live" aspect of resources, as it doesn't run Watch,
but it was designed to ensure CheckApply works as intended, and it runs
very quickly with a simplified timeline of happenings.

engine/graph/autogroup/util.go

@@ -18,6 +18,9 @@
 package autogroup
 
 import (
+	"fmt"
+
+	"github.com/purpleidea/mgmt/engine"
 	"github.com/purpleidea/mgmt/pgraph"
 	"github.com/purpleidea/mgmt/util/errwrap"
 )
@@ -112,8 +115,17 @@ func VertexMerge(g *pgraph.Graph, v1, v2 pgraph.Vertex, vertexMergeFn func(pgrap
 			// note: This branch isn't used if the vertexMergeFn
 			// decides to just merge logically on its own instead
 			// of actually returning something that we then merge.
-			v1 = v // TODO: ineffassign?
+			v1 = v // XXX: ineffassign?
 			//*v1 = *v
+
+			// Ensure that everything still validates. (For safety!)
+			r, ok := v1.(engine.Res) // TODO: v ?
+			if !ok {
+				return fmt.Errorf("not a Res")
+			}
+			if err := engine.Validate(r); err != nil {
+				return errwrap.Wrapf(err, "the Res did not Validate")
+			}
 		}
 	}
 	g.DeleteVertex(v2) // remove grouped vertex