selenium: Abstract test command behind 'npm run'

selenium: Abstract test command behind 'npm run'

This makes it possible to use in Quibble. Right now, Quibble
cannot use 'npm run selenium' yet because it sets up its own
chromedriver, which would conflict with selenium.sh.

But, in preparation for Id95f18927b5, we can at least abstract
the test command so that it can easily be changed without having
to update Quibble and without having to worry about it being the
same in all MediaWiki branches.

Bug: T179190
Change-Id: I622ec3bf36746502cae891cc6bec23982b21f876