change test file loading to be ESM-first

FIrst try ESM, and only if that fails, load test file using require.
This enables an ESM loader to have first dibs at transforming the file.

lib/esm-utils.js

@@ -34,21 +34,28 @@ exports.requireOrImport = async file => {
   if (path.extname(file) === '.mjs') {
     return formattedImport(file);
   }
-  // This is currently the only known way of figuring out whether a file is CJS or ESM.
-  // If Node.js or the community establish a better procedure for that, we can fix this code.
-  // Another option here would be to always use `import()`, as this also supports CJS, but I would be
-  // wary of using it for _all_ existing test files, till ESM is fully stable.
   try {
-    return require(file);
+    return dealWithExports(await formattedImport(file));
   } catch (err) {
-    if (err.code === 'ERR_REQUIRE_ESM') {
-      return formattedImport(file);
+    if (
+      err.code === 'ERR_MODULE_NOT_FOUND' ||
+      err.code === 'ERR_UNKNOWN_FILE_EXTENSION'
+    ) {
+      return require(file);
     } else {
       throw err;
     }
   }
 };
 
+function dealWithExports(module) {
+  if (module.default) {
+    return module.default;
+  } else {
+    return {...module, default: undefined};
+  }
+}
+
 exports.loadFilesAsync = async (files, preLoadFunc, postLoadFunc) => {
   for (const file of files) {
     preLoadFunc(file);