puppeteer_tests: Use default user test credentials for log_in().
puppeteer_tests: Use default user test credentials for log_in().

When no credentials are sent to `log_in` function we want to
use the default generated test credentials. This saves us the
work of importing test_credentials everytime we run this function
in a different test which doesn't focus on what credentials are sent
to login.

kou/zulip,shubhamdhama/zulip,showell/zulip,brainwane/zulip,showell/zulip,andersk/zulip,punchagan/zulip,timabbott/zulip,shubhamdhama/zulip,zulip/zulip,brainwane/zulip,punchagan/zulip,zulip/zulip,shubhamdhama/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,timabbott/zulip,timabbott/zulip,eeshangarg/zulip,punchagan/zulip,showell/zulip,synicalsyntax/zulip,kou/zulip,andersk/zulip,andersk/zulip,synicalsyntax/zulip,rht/zulip,hackerkid/zulip,shubhamdhama/zulip,timabbott/zulip,andersk/zulip,eeshangarg/zulip,synicalsyntax/zulip,kou/zulip,shubhamdhama/zulip,shubhamdhama/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,brainwane/zulip,showell/zulip,kou/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,timabbott/zulip,timabbott/zulip,zulip/zulip,synicalsyntax/zulip,synicalsyntax/zulip,andersk/zulip,hackerkid/zulip,synicalsyntax/zulip,brainwane/zulip,punchagan/zulip,shubhamdhama/zulip,rht/zulip,timabbott/zulip,hackerkid/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,brainwane/zulip,zulip/zulip,rht/zulip,rht/zulip,showell/zulip,showell/zulip,kou/zulip,kou/zulip