[Finishes #71693868, 71804850] couple step mode fixes

New UI tests https://github.com/code-dot-org/dashboard/pull/701

Change one of my example step levels to use stepOnly mode (so that I can UI test stepOnly mode).

Continue to highlight the last block after executing code in stepMode, so that you can easily see where you failed.

If in stepOnly mode, the run button should not appear when hitting reset.