fix(es/testing): Fix sourcemap generation (#9891)

**Description:**

This is a regression from #9150

Author: kdy1

.changeset/tricky-llamas-collect.md

@@ -0,0 +1,6 @@
+---
+swc_core: patch
+swc_ecma_transforms_testing: patch
+---
+
+fix(es/testing): Fix sourcemap generation

Cargo.lock

@@ -22,6 +22,7 @@ sha2       = { workspace = true }
 sourcemap  = { workspace = true }
 tempfile   = { workspace = true }
 
+swc_allocator = { version = "2.0.0", path = "../swc_allocator", default-features = false }
 swc_common = { version = "5.0.0", path = "../swc_common", features = [
   "sourcemap",
 ] }

crates/swc_ecma_transforms_testing/Cargo.toml

@@ -27,7 +27,7 @@ use swc_common::{
     FileName, Mark, SourceMap, DUMMY_SP,
 };
 use swc_ecma_ast::*;
-use swc_ecma_codegen::to_code_default;
+use swc_ecma_codegen::{to_code_default, Emitter};
 use swc_ecma_parser::{lexer::Lexer, Parser, StringInput, Syntax};
 use swc_ecma_testing::{exec_node_js, JsExecOptions};
 use swc_ecma_transforms_base::{
@@ -900,7 +900,7 @@ fn test_fixture_inner<'a>(
     });
 
     let mut src_map = if config.sourcemap {
-        Some(Vec::new())
+        Some(swc_allocator::maybe::vec::Vec::new())
     } else {
         None
     };
@@ -942,6 +942,24 @@ fn test_fixture_inner<'a>(
             let module = &actual;
             let comments: &Rc<SingleThreadedComments> = &tester.comments.clone();
 
+            let mut buf = vec![];
+            {
+                let mut emitter = Emitter {
+                    cfg: Default::default(),
+                    cm: tester.cm.clone(),
+                    wr: Box::new(swc_ecma_codegen::text_writer::JsWriter::new(
+                        tester.cm.clone(),
+                        "\n",
+                        &mut buf,
+                        src_map.as_mut(),
+                    )),
+                    comments: Some(comments),
+                };
+
+                // println!("Emitting: {:?}", module);
+                emitter.emit_program(module).unwrap();
+            }
+
             if let Some(src_map) = &mut src_map {
                 sourcemap = Some(tester.cm.build_source_map_with_config(
                     src_map,
@@ -950,7 +968,7 @@ fn test_fixture_inner<'a>(
                 ));
             }
 
-            to_code_default(tester.cm.clone(), Some(comments), module)
+            String::from_utf8(buf).expect("codegen generated non-utf8 output")
         };
 
         Ok(actual_src)