As puppeteer tests are similar to casper i.e both being frontend
screen scraping tests, we need to use the same webpack setup as
that of casper's instead of `build_for_most_tests` which is used
mostly for backend related tests.
This sets env variable `CASPER_TESTS` to `1` which does all the
work to make it use the same setup of casper. We would want to
rename that to `PUPPETEER_TESTS` and make changes to do the
same as casper webpack setup when we completely replace casper
with puppeteer.
Anders Kaseorg
Dinesh
Puneeth Chaganti