merge_pr_48595
[wdspec] Add wdspec tests for image cache network requests
Depends on D224505
Cross browser tests are added for basic image cache scenarios.
An additional mozilla specific wdspec test is added to cover scenarios where the behavior is implementation defined.
Differential Revision: https://phabricator.services.mozilla.com/D224506
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1886518
gecko-commit: f22ae80101e8c95fad6918a2d8d941d7413242b1
gecko-reviewers: webdriver-reviewers, Sasha