Bug 1925468 - Extend and improve modify-attributes-in-callback.html. …

…r=smaug

- Use cleanup functions to guarantee the state of the DOM tree before
  each test.
- Make more explicit why the length of the div's attribute list is
  expected to be 2.
- Add 2 tests for https://g-issues.chromium.org/issues/333739948
  using setAttributeNS instead of setAttribute.

Differential Revision: https://phabricator.services.mozilla.com/D228776

UltraBlame original commit: 094b1d5348abe71e660808ee868437dea33d2b5f