BitAutoDataAttribute is used to mark a Theory as autopopulating
parameters.
Extract common attribute methods to to a helper class. Cannot
inherit a common base, since both require inheriting from different
Xunit base classes to work.
https://github.com/AutoFixture/AutoFixture/pull/1164 shows that only
the first test case is pulled for this attribute.
This is a workaround that populates the provided parameters, left to
right, using AutoFixture to populate any remaining.
