Bug 2043893 - Redirect langpacks URL to local server in browser-srd.toml tests. r=mstriemer

This is the same issue as Bug 2041391, where langpacks are only loaded
in non-Nightly builds and so the issue was not caught in the regressing
bug, Bug 2037667. This is because of the
`intl.multilingual.downloadEnabled` pref being set to "true" in
`RELEASE_OR_BETA` and not DevEdition builds, see `firefox.js` for more
details.

Because of the SRD related changes, the languages pane will eagerly
fetch remote langpacks as soon as the pane becomes active. This fetch is
gated on `intl.multilingual.downloadEnabled`, so the test always passes
on Nightly since the pref is always false on Nightly.

As for why this was not an issue before SRD, the remote langpacks fetch
would only occur when the `browserLanguages.js` dialog was opened, not
when the pane itself was loaded. `browser_checkspelling.js` never opens
the Browser Languages dialog, so this was never an issue with the legacy
settings setup.

Differential Revision: https://phabricator.services.mozilla.com/D304194

Author: TGiles

browser/components/preferences/tests/browser-srd.toml

@@ -6,6 +6,7 @@ prefs = [
   "browser.backup.archive.enabled=false",
   "browser.backup.restore.enabled=false",
   "browser.settings-redesign.enabled=true",
+  "extensions.getAddons.langpacks.url=https://mochi.test:8000/langpacks",
 ]
 support-files = [
   "head.js",