MAINT: refactor selenium tests #113

The selenium test cases are becoming increasingly complex and difficult to read due to the large number of test cases and requisite parameters passed between the helpers and the cases that are being rendered. I'd like to refactor all of these to split tests into three categories:

• method/helper tests (i.e. python only)
• vega spec tests (i.e. what was passed from python into the rendered vega spec)
• selenium/browser tests (i.e. what was drawn where from the vega spec)

This should segment what's being tested a bit more & simplify the existing selenium tests, in addition to (hopefully) reducing run-time, as the selenium tests take a long time to run currently.