replace F by FixtureExample in auto-generated test examples to avoid documentation problems

The code contained several structs called "F", which causes all individual letters "F" in the documentation (such as abbreviations of first names in citations) automatically being linked to these structs. The affected code seems to be generated at creation of some parts of the package. Thus, renaming is not expected to require discussion. Now using "FixtureExample" instead of "F", which is also more descriptive.