Bots: disable the default browser test and enable E10s

The test runner is automated, so if the default browser test is
performed, the browser hangs waiting for user input it never gets.
Disable the test to fix that.

Moreover, enable E10s now that it is mature. This may help with the
performance of the test runner as well.

test/resources/firefox/user.js

@@ -1,3 +1,2 @@
-user_pref('browser.tabs.remote.autostart', false);
+// Disable the default browser test.
-user_pref('browser.tabs.remote.autostart.1', false);
+user_pref('browser.shell.checkDefaultBrowser', false);
-user_pref('browser.tabs.remote.autostart.2', false);