Add require() polyfill for EM_ASM tests in ESM modes

Instead of skipping EM_ASM tests that use CJS require() in ESM modes,
add a createRequire-based polyfill (available since Node 12.2.0) that
makes require() available in ESM output. The polyfill is only included
when the build targets ESM (EXPORT_ES6, MODULARIZE=instance, or
WASM_ESM_INTEGRATION).

- Add test/require_polyfill.js using createRequire from 'module'
- Add is_esm() and add_require_polyfill() helpers to test/common.py
- Remove @no_modularize_instance skips from test_fs_nodefs_rw and
  test_fs_nodefs_home, enabling them in ESM test modes

Author: Bartek Wrona

test/common.py

@@ -626,12 +626,21 @@ def require_wasm2js(self):
     if self.get_setting('WASM_ESM_INTEGRATION'):
       self.skipTest('wasm2js is not compatible with WASM_ESM_INTEGRATION')
 
+  def is_esm(self):
+    return self.get_setting('EXPORT_ES6') or self.get_setting('WASM_ESM_INTEGRATION') or self.get_setting('MODULARIZE') == 'instance'
+
+  def add_require_polyfill(self):
+    """Add a require() polyfill for ESM mode using createRequire (Node 12.2+)."""
+    if self.is_esm():
+      self.cflags += ['--pre-js', test_file('require_polyfill.js')]
+
   def setup_nodefs_test(self):
     self.require_node()
     if self.get_setting('WASMFS'):
       # without this the JS setup code in setup_nodefs.js doesn't work
       self.set_setting('FORCE_FILESYSTEM')
     self.cflags += ['-DNODEFS', '-lnodefs.js', '--pre-js', test_file('setup_nodefs.js'), '-sINCOMING_MODULE_JS_API=onRuntimeInitialized']
+    self.add_require_polyfill()
 
   def setup_noderawfs_test(self):
     self.require_node()

test/require_polyfill.js

@@ -0,0 +1,7 @@
+// Polyfill require() for ESM mode so that EM_ASM/EM_JS code using
+// require('fs'), require('path'), etc. works in both CJS and ESM.
+// createRequire is available since Node 12.2.0.
+if (typeof require === 'undefined') {
+  var { createRequire } = await import('module');
+  var require = createRequire(import.meta.url);
+}

test/test_core.py

@@ -5825,6 +5825,8 @@ def test_fs_base(self):
   def test_fs_nodefs_rw(self):
     if not self.get_setting('NODERAWFS'):
       self.setup_nodefs_test()
+    else:
+      self.add_require_polyfill()
     self.maybe_closure()
     self.do_runf('fs/test_nodefs_rw.c', 'success')
 
@@ -5848,6 +5850,7 @@ def test_fs_nodefs_dup(self):
 
   @requires_node
   def test_fs_nodefs_home(self):
+    self.add_require_polyfill()
     self.do_runf('fs/test_nodefs_home.c', 'success', cflags=['-sFORCE_FILESYSTEM', '-lnodefs.js'])
 
   @requires_node
@@ -8662,8 +8665,10 @@ def test(assert_returncode=0):
         js = read_file(self.output_name('test_hello_world.support'))
       else:
         js = read_file(self.output_name('test_hello_world'))
-      # In ESM mode, we use dynamic import() instead of require() for node modules
-      if self.get_setting('WASM_ESM_INTEGRATION'):
+      # In ESM mode we use dynamic import() instead of require() for node modules.
+      # MODULARIZE=instance implies EXPORT_ES6 which triggers ESM output.
+      is_esm = self.get_setting('EXPORT_ES6') or self.get_setting('WASM_ESM_INTEGRATION') or self.get_setting('MODULARIZE') == 'instance'
+      if is_esm:
         has_node_imports = 'import(' in js
       else:
         has_node_imports = 'require(' in js