Valera/lti graded additional tests 2

- Includes new and updated Python tests for the LTI module.
- Includes fix for <b>BLD-465</b>: e-reader error when popping out window.

<b>Description of BLD-465</b>
The attached error is seen when attempting to pop out the e-reader in a different window - happens about 50% of the time. Can be seen in 2.03x here: https://courses.edx.org/courses/MITx/2.03x/3T2013/courseware/Course_Text/Textbook/# . Also noted in 4.605x course.

<b>Note</b>
Original PR is https://github.com/edx/edx-platform/pull/1804 .

<b>Reviewers</b>
- @singingwolfboy (Python part)
- @jmclaus (JavaScript part)