All merge requests failing in tests with ProcessSearchError

Failures currently all look like this:

ubuntuuitoolkit.tests.test_emulators.TabsTestCase.test_switch_to_tab_by_object_name (from )

_StringException: Empty attachments:
  process-stdout

process-return-code: {{{1}}}
process-stderr: {{{qmlscene: could not find a Qt installation of ''}}}
test-log: {{{
08:44:38.916 INFO globals:49 - ************************************************************
08:44:38.916 INFO globals:50 - Starting test ubuntuuitoolkit.tests.test_emulators.TabsTestCase.test_switch_to_tab_by_object_name
08:44:38.916 DEBUG globals:148 - Starting: ['/usr/bin/recordmydesktop', '--no-sound', '--no-frame', '-o', '/var/local/autopilot//artifacts/ubuntuuitoolkit.tests.test_emulators.TabsTestCase.test_switch_to_tab_by_object_name.ogv']
08:44:38.966 INFO __init__:157 - Launching process: ['/usr/bin/qmlscene', '-testability', '-I/usr/lib/modules', '/tmp/tmpj3sEAz.qml', '--desktop_file_hint=/home/ubuntu/.local/share/applications/tmpYSfbAf.desktop']
08:44:40.029 INFO testcase:552 - waiting for process to exit.
08:44:40.029 INFO testcase:554 - Killing process 7647
08:44:40.029 INFO testcase:557 - Appears process has already exited.
}}}

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/ubuntuuitoolkit/tests/__init__.py", line 79, in setUp
    self.launch_application()
  File "/usr/lib/python2.7/dist-packages/ubuntuuitoolkit/tests/__init__.py", line 92, in launch_application
    app_type='qt')
  File "/usr/lib/python2.7/dist-packages/autopilot/testcase.py", line 284, in launch_test_application
    dbus_bus
  File "/usr/lib/python2.7/dist-packages/autopilot/introspection/__init__.py", line 188, in get_autopilot_proxy_object_for_process
    dbus_bus=dbus_bus,
  File "/usr/lib/python2.7/dist-packages/autopilot/introspection/__init__.py", line 263, in get_proxy_object_for_existing_process
    process
  File "/usr/lib/python2.7/dist-packages/autopilot/introspection/__init__.py", line 293, in _get_dbus_addresses_from_search_parameters
    % return_code
ProcessSearchError: Process exited with exit code: 1

This happens even with https://code.launchpad.net/~elopio/ubuntu-ui-toolkit/base_autopilot_class/+merge/185170 which is in trunk merged into the affected branches.