Improve runtime unit tests code coverage

[sameo]

Runtime's unit testing only covers 35% of the runtime code today.
The code will need some refactoring but this needs to be improved.

[jodh-intel]

This is becoming a placeholder for sub-tasks. See:

• switch unit tests to using coverage-instrumented cc-runtime binary #359
• Handle test coverage for non-root and root tests #262
• Run functional tests with a coverage-instrumented binary #191

[jodh-intel]

This issue can be used primarily to add the basic mock testing framework which will enable the other files in the runtime to have full unit tests created.

[jodh-intel]

See #394, #395, #396, #397, #398, #399, #400, #401, #402, #403, #404, #405, #406.

[jodh-intel]

Overall unit-test coverage on the runtime is now 97% as shown by Coveralls:

• https://coveralls.io/github/clearcontainers/runtime

We have plans to improve how we track changing unit-test figures, but I think we can consider the task on this issue complete.