Simple provisioning of extension browser-test dependencies

Refactored browsertests class to allow for provisioning of MediaWiki
extension browser-test dependencies without installing qa/browsertests
itself. Resources related to the latter were moved to
role::browsertests.

Added a browser_test parameter to mediawiki::extension for specification
of whether browser-test dependencies should be provisioned and, if
applicable, an alternate relative path to the extension's tests
directory.

Modified visualeditor role to provision its browser-test dependencies.

Change-Id: I363e2add582602eea7975fcd2559f3faebd22149