[web] Export stub test extension file. (Resolves #2895)

[angelosilvestre]

[web] Export stub test extension file. (Resolves #2895)

We export some test extensions in the lib folder, including code related to golden tests. This breaks the build with the following build error:

Target dart2js failed: ProcessException: Process exited abnormally with exit code 1:
../../.pub-cache/hosted/pub.dev/super_editor-0.3.0-dev.45/lib/src/test/flutter_extensions/test_tools_goldens.dart:236:55:
Error: The getter 'basedir' isn't defined for the type 'LocalFileComparator'.
 - 'LocalFileComparator' is from 'package:flutter_test/src/_goldens_web.dart'
 ('../../flutter/packages/flutter_test/lib/src/_goldens_web.dart').
        (goldenFileComparator as LocalFileComparator).basedir.path,
                                                      ^^^^^^^
../../.pub-cache/hosted/pub.dev/super_editor-0.3.0-dev.45/lib/src/test/flutter_extensions/test_tools_goldens.dart:256:14:
Error: Too many positional arguments: 0 allowed, but 1 found.
        super(Uri.parse(testBaseDirectory));
             ^
../../.pub-cache/hosted/pub.dev/super_editor-0.3.0-dev.45/lib/src/test/flutter_extensions/test_tools_goldens.dart:291:11:
Error: The method 'generateFailureOutput' isn't defined for the type 'PixelDiffGoldenComparator'.
 - 'PixelDiffGoldenComparator' is from 'package:super_editor/src/test/flutter_extensions/test_tools_goldens.dart'
 ('../../.pub-cache/hosted/pub.dev/super_editor-0.3.0-dev.45/lib/src/test/flutter_extensions/test_tools_goldens.dart').
    await generateFailureOutput(result, golden, basedir);
          ^^^^^^^^^^^^^^^^^^^^^
Error: Compilation failed.

To fix that. I modified the file that exports these extensions to export a stub file when building for web.