Speed up the tests further, and remove interdependencies between tests.

Several of the tests would call other tests to set up data, invoking API
requests to provide the data, which added to the test execution time.
This also made it harder to separate tests out into list/item suites.

The tests are now setting up their data using the new factory methods
instead of calling other test functions.
Remove leading and trailing blank lines in review comments.

When a user submits a review comment, leading and trailing
blank lines are saved as part of the comment. We want to strip
these blank lines before saving the comment to the database.

Testing done:
Created new review request and manually tested:

-review
-review reply
-diff comment
-reply diff comment
-file attachment comment
-reply file attachment comment

Unit tests pass.

Reviewed at https://reviews.reviewboard.org/r/4561/
Fixes issue 2933