Fix tests that require pkg_resources

[mdickinson]

Three of the tests require the pkg_resources package to be present in the environment. It usually will be (since it's a part of the setuptools distribution), but if it's not, then the tests should be skipped rather than causing the test run to fail.

This PR fixes that, adding pkg_resources to the existing optional imports machinery in traits.testing.

Part of the motivation here is to be able to create a test run that validates that Traits doesn't have any "accidental" dependencies. See #1299.

Closes #1300.

[mdickinson]

I tested this locally with the following steps, executed from the top level of a clean clone:

% python -m venv --clear ~/.venvs/traits
% source ~/.venvs/traits/bin/activate
% python -m pip install --upgrade pip setuptools wheel
% python -m pip install .
% python -m pip uninstall setuptools wheel
% mkdir temp
% cd temp
% python -m unittest discover -v traits

[rahulporuri]

LGTM assuming CI is also happy