tbb-testsuite: fix the searchengines test

The searchengines test is currently failing with the error:

Traceback (most recent call last):
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_harness-4.3.0-py2.7.egg/marionette_harness/marionette_test/testcases.py", line 156, in run
testMethod()
File "/home/tbb-testsuite/tbb-testsuite/marionette/tor_browser_tests/test_searchengines.py", line 25, in test_searchengines
self.marionette.navigate('about:')
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/marionette.py", line 1632, in navigate
self._send_message("get", {"url": url})
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/decorators.py", line 23, in _
return func(*args, **kwargs)
File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/marionette.py", line 740, in _send_message
    self._handle_error(err)
  File "/home/tbb-testsuite/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_driver-2.5.0-py2.7.egg/marionette_driver/marionette.py", line 764, in _handle_error
    raise errors.lookup(error)(message, stacktrace=stacktrace)
UnknownException: [Exception... "The URI is malformed"  nsresult: "0x804b000a (NS_ERROR_MALFORMED_URI)"  location: "JS frame :: chrome://marionette/content/listener.js :: get/< :: line 1053"  data: no]
stacktrace:
get/<@chrome://marionette/content/listener.js:1053:7
navigate/<@chrome://marionette/content/listener.js:415:13
navigate@chrome://marionette/content/listener.js:414:13
get@chrome://marionette/content/listener.js:1052:5