tbb-testsuite: fix the svg-disable and svg-enable tests

Running the svg-disable test currently fails 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_svg.py", line 32, in test_svg
m.navigate("%s/img_src_url.html" % self.svg_dir)
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: Reached error page: about:neterror?e=fileAccessDenied&u=file%3A///home/tbb-testsuite/tbb-testsuite/test-data/svg//img_src_url.html&c=UTF-8&f=regular&d=%D8%A7%D9%84%D9%85%D9%84%D9%81%20%D9%81%D9%8A%20/home/tbb-testsuite/tbb-testsuite/test-data/svg//img_src_url.html%20%D9%84%D9%8A%D8%B3%20%D9%85%D9%82%D8%B1%D9%88%D8%A1%D9%8B%D8%A7.
stacktrace:
	WebDriverError@chrome://marionette/content/error.js:178:5
	UnknownError@chrome://marionette/content/error.js:529:5
handleReadyState@chrome://marionette/content/listener.js:277:21
handleEvent@chrome://marionette/content/listener.js:245:9

The svg-enable test fails with the same 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_svg.py", line 32, in test_svg
m.navigate("%s/img_src_url.html" % self.svg_dir)
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: Reached error page: about:neterror?e=fileAccessDenied&u=file%3A///home/tbb-testsuite/tbb-testsuite/test-data/svg//img_src_url.html&c=UTF-8&f=regular&d=%D8%A7%D9%84%D9%85%D9%84%D9%81%20%D9%81%D9%8A%20/home/tbb-testsuite/tbb-testsuite/test-data/svg//img_src_url.html%20%D9%84%D9%8A%D8%B3%20%D9%85%D9%82%D8%B1%D9%88%D8%A1%D9%8B%D8%A7.
stacktrace:
	WebDriverError@chrome://marionette/content/error.js:178:5
	UnknownError@chrome://marionette/content/error.js:529:5
handleReadyState@chrome://marionette/content/listener.js:277:21
handleEvent@chrome://marionette/content/listener.js:245:9