Bug 1865689 - Fixup browser.toml - move prefs to DEFAULT. a=test-only

The individual test ran as expected when prefs was next to the file.
But when the whole directory is selected, the test runner refuses to
run the test due to the following error:

> The 'prefs' key must be set in the DEFAULT section of a manifest.

This patch fixes the issue by moving prefs to DEFAULT. This is okay
because the other test in the test manifest is independent of the pref.

Differential Revision: https://phabricator.services.mozilla.com/D196295

devtools/shared/commands/inspected-window/tests/browser.ini

@@ -8,8 +8,12 @@ support-files =
   head.js
   inspectedwindow-reload-target.sjs
 
-[browser_webextension_inspected_window.js]
-[browser_webextension_inspected_window_access.js]
 prefs =
-  # restrictedDomains must be set as early as possible.
+  # restrictedDomains must be set as early as possible, before the first use of
+  # the preference. browser_webextension_inspected_window_access.js relies on
+  # this pref to be set. We cannot use "prefs =" at the individual file, because
+  # another test in this manifest may already have resulted in browser startup.
   extensions.webextensions.restrictedDomains=test2.example.com
+
+[browser_webextension_inspected_window.js]
+[browser_webextension_inspected_window_access.js]
\ No newline at end of file