Use IterStart to start browser process for iteration

[ka3de]

Once grafana/k6#2432 is done, k6 browser, as a JSModule, will be able to receive events from k6 core for:

• Init: when k6 starts initializing outputs, VUs and executors.
• TestStart: when the execution scheduler starts running the test.
• TestEnd: when the test execution ends.
• IterStart: when a VU starts an iteration.
• IterEnd: when a VU ends an iteration.
• Exit: when the k6 process is about to exit.

k6 browser should integrate with the IterStart event in order to pre initialize the browser process for the iteration before its execution starts, so the the browser lifecycle handling implemented in #910 is improved.