Add next-custom-transforms tests to verify source maps

This allows us to quickly verify that the spans that are defined in the
SWC transforms yield correct source maps. The `output.js` and
`output.js.map` files can be used to visualize the source mapping, e.g.
at https://evanw.github.io/source-map-visualization/.

For now I'm adding a single fixture for `'use cache'` with this PR.

crates/next-custom-transforms/tests/fixture.rs

@@ -916,6 +916,36 @@ fn test_edge_assert(input: PathBuf) {
     );
 }
 
+#[fixture("tests/fixture/source-maps/**/input.js")]
+fn test_source_maps(input: PathBuf) {
+    let output = input.parent().unwrap().join("output.js");
+    test_fixture(
+        syntax(),
+        &|_tr| {
+            (
+                resolver(Mark::new(), Mark::new(), false),
+                server_actions(
+                    &FileName::Real("/app/item.js".into()),
+                    server_actions::Config {
+                        is_react_server_layer: true,
+                        dynamic_io_enabled: true,
+                        hash_salt: "".into(),
+                        cache_kinds: FxHashSet::from_iter([]),
+                    },
+                    _tr.comments.as_ref().clone(),
+                ),
+            )
+        },
+        &input,
+        &output,
+        FixtureTestConfig {
+            module: Some(true),
+            sourcemap: true,
+            ..Default::default()
+        },
+    );
+}
+
 fn lint_to_fold<R>(r: R) -> impl Pass
 where
     R: Visit,

crates/next-custom-transforms/tests/fixture/source-maps/use-cache/1/input.js

@@ -0,0 +1,9 @@
+'use cache'
+
+const foo = async () => {
+  'use cache'
+}
+
+export async function bar() {
+  return foo()
+}