core: Update expected error message for plan countComputedModule test

Previously we only handled the "count cannot be computed" check during
validate, leaving other walks to just report "a number is required"
(because "unknown" was represented as a special string) but now we have
unknown as first-class we handle it during all walks, and so this error
message is now the more appropriate one saying that the value is not
yet known.

terraform/context_plan_test.go

@@ -1758,7 +1758,7 @@ func TestContext2Plan_countComputedModule(t *testing.T) {
 
 	_, err := ctx.Plan()
 
-	expectedErr := "a number is required"
+	expectedErr := `The "count" value depends on resource attributes`
 	if !strings.Contains(fmt.Sprintf("%s", err), expectedErr) {
 		t.Fatalf("expected err would contain %q\nerr: %s\n",
 			expectedErr, err)