Add a codesize test that includes file packager output. NFC (#24991)

Include the full generated JS so that changes to file_packager itself
get reflected in the output here.

Author: sbc100

test/code_size/test_codesize_file_preload.json

@@ -0,0 +1,28 @@
+{
+  "a.out.js": 22730,
+  "a.out.js.gz": 9405,
+  "a.out.nodebug.wasm": 1733,
+  "a.out.nodebug.wasm.gz": 980,
+  "total": 24463,
+  "total_gz": 10385,
+  "sent": [
+    "a (fd_write)"
+  ],
+  "imports": [
+    "a (fd_write)"
+  ],
+  "exports": [
+    "b (memory)",
+    "c (__wasm_call_ctors)",
+    "d (main)"
+  ],
+  "funcs": [
+    "$__emscripten_stdout_close",
+    "$__emscripten_stdout_seek",
+    "$__fwritex",
+    "$__stdio_write",
+    "$__towrite",
+    "$__wasm_call_ctors",
+    "$main"
+  ]
+}