test_ext_dns.js is currently skipped on Android.

When I run it, the test times out without notable errors, but adb logcat does show:

[JavaScript Error: "Error: proxy.settings is not supported on android." {file: "undefined" line: 0}]

This error is triggered by the test at: https://searchfox.org/mozilla-central/rev/4b88e0b8cca115009e82fdd65e5bf5812ff99128/toolkit/components/extensions/test/xpcshell/test_ext_dns.js#26

The proxy.settings API should either be supported on Android, or the test should be rewritten to not depend on this API (e.g. by using alternative APIs or skipping the part of the test that tests proxy behavior).

We should re-enable this test since it is the only form of test coverage for the browser.dns API, and the API is used by the most popular add-on on our Firefox for Android browser (uBlock origin uses this API to counter CNAME cloaking).

The DNS API is used by uBlock Origin, so it’s important to support. The tests aren’t enable because one test case uses proxy, which we don’t support yet on Android. For M2, we can disable the proxy test case and run all the other test cases.