This test ensures selectstart is fired exactly once when selecting text by a mouse drag.

Initial state: PASS
Mouse down: PASS
Mouse up: PASS
Second mouse down: FAIL - expected selection to be range but was caret
Second mouse up: FAIL - expected selection to be range but was caret
Third mouse down: FAIL - expected selection to be range but was caret
Third mouse up: FAIL - expected selection to be range but was caret
Done.