puppeteer_tests: Add missing `waitFor` call to avoid rare flake.
puppeteer_tests: Add missing `waitFor` call to avoid rare flake.

This commit fixes a rare flake where it accidentally clicks
on "Subscribe to more streams"  instead of "Verona"
(stream). This happens because `#stream_filters` list
was not completely updated, i.e. It still had "Venice" and
"Verona" on the list, when it searched for "ver".
When it decided to click on the "Verona", "Venice"
disappeared, which causes "Verona" to move up and
causing "Subscribe to more streams" to click.

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