states: In Module.testString, use incrementing ids for deposed

In our old world we always used 1-based indices into a slice of deposed
objects. The new models instead use a map keyed by pseudorandom strings,
so that deposed objects will have a consistent identity across multiple
operations.

However, having that pseudo-random string in our test comparison output
is not helpful, since such strings can never match hard-coded expectation
strings. Therefore for the purposes of generating this test comparison
output we'll revert back to using 1-based indexes.

This should avoid problems for tests that only create one deposed object
per instance, but those which create more than one will need to do some
more work since the _ordering_ of these objects in the output is still
pseudorandom as a result of it coming from a map rather than a slice.

states/state_string.go

@@ -182,13 +182,15 @@ func (m *Module) testString() string {
 		// CAUTION: Since deposed keys are now random strings instead of
 		// incrementing integers, this result will not be deterministic
 		// if there is more than one deposed object.
-		for k, t := range is.Deposed {
+		i := 1
+		for _, t := range is.Deposed {
 			id := legacyInstanceObjectID(t)
 			taintStr := ""
 			if t.Status == ObjectTainted {
 				taintStr = " (tainted)"
 			}
-			buf.WriteString(fmt.Sprintf("  Deposed ID %s = %s%s\n", k, id, taintStr))
+			buf.WriteString(fmt.Sprintf("  Deposed ID %d = %s%s\n", i, id, taintStr))
+			i++
 		}
 
 		if obj := is.Current; obj != nil && len(obj.Dependencies) > 0 {