Fix bug in wasm preload plugin (#19342)

Because we were not passing `byteArray` to the onload handle, no data
was actually getting written to the fileystem in FS_createPreloadedFile
-> processData -> finish and these preloaded files were being written as
empty files.

This matches the behavior of the other two plugins we have for images
and audio.

Fixing this bug allows us to fix one of the issues with
`test_preload_module`, which is that it was falling back to loading from
the a URL if the file was not found.  By preloading the file under a
different name we prevent the URL fallback from working.  The URL
fallback had been silently masking the `FS_createPreloadedFile` issue.

Author: sbc100

src/library_dylink.js

@@ -27,7 +27,7 @@ var LibraryDylink = {
           () => loadWebAssemblyModule(byteArray, {loadAsync: true, nodelete: true})).then(
             (module) => {
               preloadedWasm[name] = module;
-              onload();
+              onload(byteArray);
             },
             (error) => {
               err('failed to instantiate wasm: ' + name + ': ' + error);

test/test_other.py

@@ -13425,14 +13425,21 @@ def test_preload_module(self, args):
         return 42;
       }
     ''')
-    self.run_process([EMCC, 'library.c', '-sSIDE_MODULE', '-o', 'library.so'] + args)
+    self.run_process([EMCC, 'library.c', '-sSIDE_MODULE', '-o', 'tmp.so'] + args)
     create_file('main.c', r'''
       #include <assert.h>
       #include <dlfcn.h>
       #include <stdio.h>
+      #include <sys/stat.h>
       #include <emscripten.h>
       #include <emscripten/threading.h>
+
       int main() {
+        // Check the file exists in the VFS
+        struct stat statbuf;
+        assert(stat("/library.so", &statbuf) == 0);
+
+        // Check that it was preloaded
         if (emscripten_is_main_runtime_thread()) {
           int found = EM_ASM_INT(
             return preloadedWasm['/library.so'] !== undefined;
@@ -13449,4 +13456,4 @@ def test_preload_module(self, args):
         return 0;
       }
     ''')
-    self.do_runf('main.c', 'done\n', emcc_args=['-sMAIN_MODULE=2', '--preload-file', '.@/', '--use-preload-plugins'] + args)
+    self.do_runf('main.c', 'done\n', emcc_args=['-sMAIN_MODULE=2', '--preload-file', 'tmp.so@library.so', '--use-preload-plugins'] + args)