Test it is on every control: on the unit screen, in the testing panels, on the checklist page and on Control Criticality. It takes you to the quickest sensible place to test that control.
What it does
If the control already has a test on the board: it opens that test.
If not, and its scheduled test is due: it starts the scheduled test.
If not, and nothing is due: it starts an extra test, which never moves the regular schedule.
Then:
if you can answer checks at the test's stage, the Test Runner opens;
otherwise the Activity Board opens with the card picked, so you can move it on.
Test it never moves a test through the workflow for you. It only starts it and takes you there.