puppeteer: Save pid of `run-dev.py --test` in var/puppeteer.
puppeteer: Save pid of `run-dev.py --test` in var/puppeteer.

Also replaces a reference to casper in a comment related to --test.

hackerkid/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,kou/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,zulip/zulip,rht/zulip,zulip/zulip,rht/zulip,punchagan/zulip,andersk/zulip,zulip/zulip,kou/zulip,kou/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,showell/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,showell/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,showell/zulip,rht/zulip,showell/zulip,showell/zulip,showell/zulip,kou/zulip,zulip/zulip