Bug 1594878 - Keep Fission processes alive in browser/components/uitour/test/. r=kmag

There are a lot of intermittent hangs in this directory when Fission
is enabled. Keeping processes alive should avoid these.

Differential Revision: https://phabricator.services.mozilla.com/D52389

--HG--
extra : moz-landing-system : lando

browser/components/uitour/test/browser.ini

@@ -1,4 +1,9 @@
 [DEFAULT]
+prefs =
+    # With Fission enabled, this directory spawns many child processes, which
+    # somehow results in frequent leaks shutdown hangs. Reuse the content
+    # processes to avoid this.
+    dom.ipc.keepProcessesAlive.webIsolated.perOrigin=1
 support-files =
   head.js
   image.png