Use unittest discover for running tests

[ievacerny]

Closes #514

The PR adds a custom test loader functions that is used by unittest discover which allows excluding tests that match with a regex pattern specified through environment variable EXCLUDE_TESTS.

This is an attempt to simulate the behaviour of --exclude flag in nosetests.

Caveat: unittest attempts to import all test modules during loading, therefore, it will try to import test modules in ui/qt4 when running tests for wx. The import will fail and the loader will return a unittest.loader._FailedTest which has the path to the test module in its ID. Because the path contains excluded pattern qt the test will be filtered out.

[ievacerny]

I've been running the test suite in many different ways to check if the behaviour of unittest and nose is the same, but I'm struggling to get consistent results.

It might be due to test interactions. Investigating it now but it might take a while as it's not an easy thing to debug.

[corranwebster]

Looks good, and seems equivalent to what nose does, so good from that point of view. We need to fix the imports done in pyface/__init__.py as they will be imported whenever pyface is imported for the first time as a side-effect.

I have some concerns about the "collect and then filter" approach, particularly if we are running the tests in an environment with both wxPython and a Qt installed. If we get unlucky in that situation then we may end up with a Qt app (or vice-versa a Wx app) up and running before the backend we really want gets initialized. But we can prevent that by importing pyface.toolkit as an early line in load_tests.

It might even be possible to use the pyface.toolkit value to select the filter rather than having to rely on an environment variable (but maybe still have the choice overridable by an envirionment variable for safety, if you do this). But isn't needed for this PR.

[corranwebster]

Wx test failures are because:

1. the linux wheel referenced in etstool.py is no longer there, and we need another one
2. pip is installing wxPython 4.1 on mac and it has introduced test failures

[corranwebster]

With #510 merged, this needs a merge of master which will hopefully then make the tests pass.

[codecov-io]

Codecov Report

Merging #515 into master will increase coverage by 0.11%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
+ Coverage   36.84%   36.95%   +0.11%     
==========================================
  Files         470      470              
  Lines       25921    25951      +30     
  Branches     3931     3937       +6     
==========================================
+ Hits         9550     9590      +40     
+ Misses      15955    15943      -12     
- Partials      416      418       +2     

Impacted Files	Coverage Δ
pyface/ui/qt4/workbench/split_tab_widget.py	12.47% <50.00%> (ø)
pyface/__init__.py	82.60% <87.50%> (+11.18%)	⬆️
pyface/qt/QtCore.py	100.00% <100.00%> (ø)
pyface/qt/QtGui.py	100.00% <100.00%> (ø)
pyface/qt/QtMultimedia.py	100.00% <100.00%> (ø)
pyface/qt/QtMultimediaWidgets.py	100.00% <100.00%> (ø)
pyface/qt/QtNetwork.py	100.00% <100.00%> (ø)
pyface/qt/QtOpenGL.py	100.00% <100.00%> (ø)
pyface/qt/QtSvg.py	100.00% <100.00%> (ø)
pyface/qt/QtTest.py	100.00% <100.00%> (ø)
... and 13 more

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[ievacerny]

The tests on Travis seem to work fine so I'm merging this PR...