puppeteer_test: Add `is_firefox` option to enable/disable some tests.
puppeteer_test: Add `is_firefox` option to enable/disable some tests.

This commit introduces the `is_firefox`
option in CommonUtils to identify if a browser
is Firefox or not.

This helps us to avoid calling some tests
that is yet not compatible with Firefox.

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