[web] Clean up duplicate golden methods

[mdebbar]

While working on #36095 I realized we have several golden-related helpers that are duplicated throughout our tests. This PR attempts to clean up the duplicates.

• matchSceneGolden()
• matchPictureGolden()
• canvasScreenshot()
• checkScreenshot()
• setUpStableTestFonts()

As a future effort, we may want to unify some of these helpers.

[skia-gold]

Gold has detected about 2 new digest(s) on patchset 1.
View them at https://flutter-engine-gold.skia.org/cl/github/36122

[yjbanov]

[skia-gold]

Gold has detected about 2 new digest(s) on patchset 2.
View them at https://flutter-engine-gold.skia.org/cl/github/36122

[skia-gold]

Gold has detected about 1 new digest(s) on patchset 3.
View them at https://flutter-engine-gold.skia.org/cl/github/36122

[skia-gold]

Gold has detected about 2 new digest(s) on patchset 4.
View them at https://flutter-engine-gold.skia.org/cl/github/36122

[flutter-dashboard]

Golden file changes have been found for this pull request. Click here to view and triage (e.g. because this is an intentional change).

If you are still iterating on this change and are not ready to resolve the images on the Flutter Gold dashboard, consider marking this PR as a draft pull request above. You will still be able to view image results on the dashboard, commenting will be silenced, and the check will not try to resolve itself until marked ready for review.

Changes reported for pull request #36122 at sha 844dc1b