make browsertest runner a macro

Make the browsertest runner shell script a macro "browsertest" which
require the parameter 'tests-path'. I will reuse that macro in a project
used to test VisualEditor and Parsoid.

Also moved the /data creation and permissions fix to the macro. They are
tightly coupled anyway.

Change-Id: Ie0b65bd519ead9459e947c0133c415df2cd87ff0