Improve test coverage for the StructureDirectory class

[brettvanderwerff]

We used to have good coverage for the methods of the StructureDirectory class in the structure_directory.py module. Most of these tests relied on comparing the output of the example "Alstar" Bootstrap template being processed via the Flaskerizer to "gold standard" versions of these output files line by line via assertMultiLineEqual

Because of the recent PR #54 producing the issue #55, this comparison of output to gold standards approach no longer works until #55 is resolved.

Regardless, some better testing coverage for the functions in the StructureDirectory class would be a good addition.

[brettvanderwerff]

I'm working on this with PR #61

[brettvanderwerff]

This was partially solved with PR #61, but some methods remain untested and could use some tests