core: Fix TestContext2Plan_dataResourceBecomesComputed

Now that ProposedNewState uses null to represent Computed attributes not
set in the configuration, the provider must fill in the unknown value for
"computed" in its plan result.

It seems that this test was incorrectly updated during our bulk-fix after
integrating the HCL2 work, but it didn't really matter because the
ReadDataSource function isn't called in the happy path anyway. But to
make the intent clearer here, we also now make ReadDataSource return an
error if it is called, making it explicit that no call is expected.

terraform/context_plan_test.go

@@ -1985,8 +1985,12 @@ func TestContext2Plan_dataResourceBecomesComputed(t *testing.T) {
 	}
 
 	p.PlanResourceChangeFn = func(req providers.PlanResourceChangeRequest) providers.PlanResourceChangeResponse {
+		fooVal := req.ProposedNewState.GetAttr("foo")
 		return providers.PlanResourceChangeResponse{
-			PlannedState:   req.ProposedNewState,
+			PlannedState: cty.ObjectVal(map[string]cty.Value{
+				"foo":      fooVal,
+				"computed": cty.UnknownVal(cty.String),
+			}),
 			PlannedPrivate: req.PriorPrivate,
 		}
 	}
@@ -1995,9 +1999,9 @@ func TestContext2Plan_dataResourceBecomesComputed(t *testing.T) {
 	ty := schema.ImpliedType()
 
 	p.ReadDataSourceResponse = providers.ReadDataSourceResponse{
-		State: cty.ObjectVal(map[string]cty.Value{
-			"foo": cty.UnknownVal(cty.String),
-		}),
+		// This should not be called, because the configuration for the
+		// data resource contains an unknown value for "foo".
+		Diagnostics: tfdiags.Diagnostics(nil).Append(fmt.Errorf("ReadDataSource called, but should not have been")),
 	}
 
 	ctx := testContext2(t, &ContextOpts{