test: Improve `template_not_found` test to detect stable/nightly mismatch error

cc #50

sailfish-tests/integration-tests/tests/fails/template_not_found.rs

@@ -1,6 +1,10 @@
 use sailfish::TemplateOnce;
 use sailfish_macros::TemplateOnce;
 
+#[derive(TemplateOnce)]
+#[template(path = "empty.stpl")]
+struct ExistTemplate;
+
 #[derive(TemplateOnce)]
 #[template(path = "not_exist.stpl")]
 struct NotExistTemplate {
@@ -8,5 +12,6 @@ struct NotExistTemplate {
 }
 
 fn main() {
+    println!("{}", ExistTemplate.render_once().unwrap());
     println!("{}", NotExistTemplate { var: 1996 }.render_once().unwrap());
 }

sailfish-tests/integration-tests/tests/fails/template_not_found.stderr

@@ -1,26 +1,18 @@
 error: Template file "not_exist.stpl" not found
- --> $DIR/template_not_found.rs:5:19
+ --> $DIR/template_not_found.rs:9:19
   |
-5 | #[template(path = "not_exist.stpl")]
+9 | #[template(path = "not_exist.stpl")]
   |                   ^^^^^^^^^^^^^^^^
 
 error[E0599]: no method named `render_once` found for struct `NotExistTemplate` in the current scope
-  --> $DIR/template_not_found.rs:11:51
+  --> $DIR/template_not_found.rs:16:51
    |
-6  | struct NotExistTemplate {
+10 | struct NotExistTemplate {
    | ----------------------- method `render_once` not found for this
 ...
-11 |     println!("{}", NotExistTemplate { var: 1996 }.render_once().unwrap());
+16 |     println!("{}", NotExistTemplate { var: 1996 }.render_once().unwrap());
    |                                                   ^^^^^^^^^^^ method not found in `NotExistTemplate`
    |
    = help: items from traits can only be used if the trait is implemented and in scope
    = note: the following trait defines an item `render_once`, perhaps you need to implement it:
            candidate #1: `TemplateOnce`
-
-warning: unused import: `sailfish::TemplateOnce`
- --> $DIR/template_not_found.rs:1:5
-  |
-1 | use sailfish::TemplateOnce;
-  |     ^^^^^^^^^^^^^^^^^^^^^^
-  |
-  = note: `#[warn(unused_imports)]` on by default