Running the testsuite without a PyFace Toolkit shouldn't cause errors

The following errors can be seen when the testsuite is run from an environment which does not contain a PyFace Toolkit -

```
======================================================================
ERROR: test_get_engine_backend_simple_with_existing_engine (mayavi.tests.test_engine_manager.TestEngineManager)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/runner/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/unittest/mock.py", line 1183, in patched
return func(*args, **keywargs)
File "/Users/runner/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/site-packages/mayavi/tests/test_engine_manager.py", line 40, in test_get_engine_backend_simple_with_existing_engine
self.assertIs(type(get_engine()), Engine)
File "/Users/runner/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/site-packages/mayavi/tools/engine_manager.py", line 92, in get_engine
return self.new_engine()
File "/Users/runner/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/site-packages/mayavi/tools/engine_manager.py", line 137, in new_engine
check_backend()
File "/Users/runner/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/site-packages/mayavi/tools/engine_manager.py", line 40, in check_backend
raise ImportError(msg)
ImportError: Could not import backend for traitsui. Make sure you
have a suitable UI toolkit like PyQt/PySide or wxPython
installed.

======================================================================
ERROR: test_mlab_scene_model_with_gui (mayavi.tests.test_garbage_collection.TestMayaviGarbageCollection)
Tests if MlabSceneModel with GUI can be garbage collected.
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/runner/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/site-packages/mayavi/tests/test_garbage_collection.py", line 23, in test_mlab_scene_model_with_gui
class MlabApp(HasTraits):
File "/Users/runner/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/site-packages/mayavi/tests/test_garbage_collection.py", line 29, in MlabApp
editor=SceneEditor(scene_class=MayaviScene),
File "/Users/runner/hostedtoolcache/Python/3.6.15/x64/lib/python3.6/site-packages/pyface/base_toolkit.py", line 190, in __init__
raise NotImplementedError(msg % (toolkit, package, name))
NotImplementedError: the null tvtk.pyface.ui.null backend doesn't implement scene_editor:SceneEditor
```

See the output of the GitHub Actions CI job - https://github.com/enthought/mayavi/runs/3774411113?check_suite_focus=true for reference.