Text Fragments: Separate json test case to let others pass.
The test to check if scrolling is allowed in non-html documents times out for JSON,
because it depends on the onload event, which Gecko is not firing for JSON (Bug 1905106).
Additionally, other test cases in that file can time out on CI as well.
With this patch, the JSON test case is separated into its own file and is expected to TIMEOUT. The other test cases get a longer timeout so that they pass in all configurations.
Differential Revision: https://phabricator.services.mozilla.com/D216565
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1907901
gecko-commit: 72e40f9471c4f8348d84d2e211b97f26c4a84d7b
gecko-reviewers: dom-core, sefeng