Use mixin instead of having ProviderExtensionRegistryTestCase inherit from ExtensionRegistryTestCase

[aaronayres35]

fixes #281

This PR pulls out methods from test_extension_registry.ExtensionRegistryTestCase into a mixin class called test_extension_registry_mixin.ExtensionRegistryTestMixin which both ExtensionRegistryTestCase and ProviderExtensionRegistryTestCase now inherit from.

Test methods which were previously in ExtensionRegistryTestCase and overridden in ProviderExtensionRegistryTestCase have been left in these respective test cases and not included in the mixin.

This should make it easier to add new tests to ExtensionRegistryTestCase.

I searched for other TestCase subclassing but the only other occurrence I found was EggBasedTestCase being subclassed by test_egg_plugin_manager.EggPluginManagerTestCase. This I felt was fine to leave though because EggBasedTestCase actually does not contain any tests, it simply contains helper methods of setUp, _add_egg, and _add_eggs_on_path.

Also, while working on this PR I did not see a test for https://github.com/enthought/envisage/blob/master/envisage/plugin_extension_registry.py
Such a test may want to use this mixin as well. Such a test case would really only need to additionally test the trait change handlers on the plugin_manager.

[kitchoi]

Overall looks good.

I think we can now remove this comment?:

envisage/envisage/tests/test_provider_extension_registry.py

Lines 511 to 512 in 1eccd80

	# Overriden to test differing behavior between the provider registry and
	# the base class.

[kitchoi]

LGTM

[kitchoi]

Now I am also investigating into these tests, and I realized that perhaps this test should be moved to the mixin too.
The mixin is useful for testing the compliance of the IExtensionRegistry interface: IExtensionRegistry.set_extensions does not say subclass is allowed to raise and so the assumption is that it should not raise. ProviderExtensionRegistry violates the interface requirement and makes set_extensions always raise. So its test will also have to override the test, as a penalty for violating the interface.

[kitchoi]

Maybe the test mixin class should be repurposed to restrict itself to unsettable registry behaviour...
Nothing to do here. Please ignore me.