This commit add an option to shim the whole wibox module when
running multi-screen tests. This is intended to lower the test
runtime when coverage is enabled.
In theory, most of that code is already covered by the
test-screen-changes suit.
This makes the coverage builds about twice as fast. This is
necessary because there is a large number of timeouts due to
limited resources on the Travis build system.
Emmanuel Lepage Vallee
Uli Schlachter
Daniel Hahler