Include circular diagrams in label tests (currently limited support)
Explicitly test default vs 'start' for feature labels.

This awaits a code change to see if the desired change worked.
Support feature label_position of middle/center/centre on circular diagrams

These values are already supported on the linear GenomeDiagrams,
so it makes sense to offer them on circular GenomeDiagrams too.
Thanks for David Martin for alternative implementation and sharing
a motivating usecase, and his pull request:
https://github.com/biopython/biopython/pull/116

This commit included an attempt to refactor the label placement
code to reduce code duplication and simplify further work here.
(cherry picked from commit a50a440f1e49ca08829194e11339c978db7286a3)
Support feature label_position of start/end on circular diagrams
Explicitly use default label placement in this test
Mention circular GenomeDiagram figures now support label_position in NEWS, thank David Martin