tbb-testsuite: marionette tests are failing when using strace

In the marionette tests that we run from the testsuite, we are running the browser with strace in order to check the system calls and detect possible files created outside the browser directory, and direct network connections.

When running the testsuite on an esr60 based Tor Browser, the browser window does not open and an strace process takes a lot of cpu, until we get the following timeout error:

Using workspace for temporary data: "/home/user/tbb-testsuite/reports/r/FmdSZphWEU/results-tor-browser-linux64-8.0a9_es-ES.tar.xz/slider_settings_1_ws"
mozversion application_buildid: 20180204020101
mozversion application_display_name: Tor Browser
mozversion application_id: {ec8030f7-c20a-464f-9b0e-13a3a9e97384}
mozversion application_name: Firefox
mozversion application_remotingname: firefox
mozversion application_vendor: Mozilla
mozversion application_version: 60.1.0
mozversion platform_buildid: 20180204020101
mozversion platform_version: 60.1.0
Application command: /home/user/tbb-testsuite/tmp/_VycbfqfU2/tor-browser_es-ES/Browser/ff_slider_settings_1 -no-remote -marionette -profile /home/user/tbb-testsuite/reports/r/FmdSZphWEU/results-tor-browser-linux64-8.0a9_es-ES.tar.xz/slider_settings_1_ws/profile.default-1534170518
 2:00.17 LOG: MainThread ERROR Failure during harness execution
Traceback (most recent call last):

  File "/home/user/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_harness-4.3.0-py2.7.egg/marionette_harness/runtests.py", line 92, in cli
    failed = harness_instance.run()

  File "/home/user/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_harness-4.3.0-py2.7.egg/marionette_harness/runtests.py", line 72, in run
    runner.run_tests(tests)

  File "/home/user/tbb-testsuite/virtualenv-marionette-4.3.0/local/lib/python2.7/site-packages/marionette_harness-4.3.0-py2.7.egg/marionette_harness/runner/base.py", line 837, in run_tests
    self.marionette = self.driverclass(**self._build_kwargs())

  File "/home/user/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 621, in __init__
    self.start_binary(self.startup_timeout)

  File "/home/user/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 633, in start_binary
    self.raise_for_port(timeout=timeout)

  File "/home/user/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 710, in raise_for_port
    self.host, self.port))

IOError: Process killed after 120s because no connection to Marionette server could be established. Check gecko.log for errors

Disabling the use of strace is fixing the issue.