[palette_generator] Add web support to unit tests (flutter#4440)

Updates the unit tests to run in web mode as well as VM mode, since the package supports web. To remove the file dependency, this adds base64-encoded versions of the existing test images. The originals are left as source-of-truth, in case we want to handle this differently in the future, or to visually inspect the images.

Fixes flutter#129839

Author: stuartmorgan-g

packages/palette_generator/dart_test.yaml

@@ -12,7 +12,6 @@ dependencies:
   collection: ^1.15.0
   flutter:
     sdk: flutter
-  path: ^1.8.0
 
 dev_dependencies:
   flutter_test: