core: testDiffFn must properly populate "Old" in computed attr diff

When applying a diff to a value we verify that the "old" value in diff is
consistent with the given prior value, as a safety check.

The mock must comply with this or else any tests that produce diffs with
computed new values will not pass the safety check.

This change is verified by the now-passing TestContext2Apply_taintDep .

terraform/context_test.go

@@ -296,8 +296,12 @@ func testDiffFn(
 		if k == "id" {
 			continue
 		}
+		old := ""
+		if s != nil {
+			old = s.Attributes[k]
+		}
 		diff.Attributes[k] = &ResourceAttrDiff{
-			Old:         "",
+			Old:         old,
 			NewComputed: true,
 		}
 	}