Increased test coverage

[matthewhegarty]

Problem

Code coverage showed some gaps in testing. I've increased this where possible.

Solution

• Added unit tests as required
• No functional changes have been made
• Also fixed 'lint' command in Makefile

Note that test_collect_failed_rows() was included in the 'postgres' section, hence didn't run in non-PG env. I moved this to the 'general' test section so that it will run.

Acceptance Criteria

• test suite passes

[coveralls]

Coverage increased (+0.9%) to 98.796% when pulling 8790738 on matthewhegarty:improve-code-coverage into 4167a16 on django-import-export:main.

[andrewgy8]

Fantastic @matthewhegarty !