add updated test script/assets for .webp files

[ixc7]

Summary

Adds an updated test script and test.webp image
See also #2712 - apostrophecms/apostrophe#2712
See also - apostrophecms/apostrophe#3677

Test cases

What are the specific steps to test this change?

1. Clone the repository and install dependencies
2. ./webp-test.js or npm run webp

What kind of change does this PR introduce?

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[boutell]

Conflicts require resolution.