Fix ResourceWarnings from test suite.

[mdickinson]

On Python 3.6, there are ResourceWarnings due to unclosed files reported by the test suite. For example:

test_capture_errors_on_error (pyface.ui.qt4.util.tests.test_modal_dialog_tester.TestModalDialogTester) ... /Users/mdickinson/.edm/envs/pyface-test-3.6-pyqt/lib/python3.6/site-packages/pyface-6.1.0.dev13-py3.6.egg/pyface/ui/qt4/util/tests/test_modal_dialog_tester.py:143: ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='w' encoding='UTF-8'>
  tester.open_and_run(when_opened=raise_error)
ok
test_capture_errors_on_failure (pyface.ui.qt4.util.tests.test_modal_dialog_tester.TestModalDialogTester) ... /Users/mdickinson/.edm/envs/pyface-test-3.6-pyqt/lib/python3.6/site-packages/pyface-6.1.0.dev13-py3.6.egg/pyface/ui/qt4/util/tests/test_modal_dialog_tester.py:124: ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='w' encoding='UTF-8'>
  tester.open_and_run(when_opened=failure)
ok

This PR fixes (at least some of) those warnings.

[corranwebster]

LGTM

[mdickinson]

There's an extra "not" in this docstring that shouldn't be there.

[codecov-io]

Codecov Report

Merging #326 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #326      +/-   ##
==========================================
+ Coverage   24.65%   24.67%   +0.02%     
==========================================
  Files         455      455              
  Lines       25549    25554       +5     
  Branches     3851     3850       -1     
==========================================
+ Hits         6298     6306       +8     
+ Misses      18939    18938       -1     
+ Partials      312      310       -2

Impacted Files	Coverage Δ
pyface/ui/qt4/util/testing.py	67.69% <100%> (+7.69%)	⬆️

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[mdickinson]

@corranwebster Thanks! Merging in spite of the fact that you haven't approved the last commit ...