Add results and description testset methods

[mmiller-max]

Closes #3 by defining the abstract type methods in TestReports.

[codecov]

Codecov Report

Merging #71 (0f189ac) into master (d4ebb32) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #71      +/-   ##
==========================================
+ Coverage   91.70%   91.89%   +0.18%     
==========================================
  Files           6        6              
  Lines         398      407       +9     
==========================================
+ Hits          365      374       +9     
  Misses         33       33              

Impacted Files	Coverage Δ
src/testsets.jl	99.09% <100.00%> (+0.08%)	⬆️
src/to_xml.jl	100.00% <100.00%> (ø)

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[oxinabox]

On further thoughts I am not sure if these are required.
Should we table this PR until someone requests it?

Looks fine to me, but maybe it is premature gold plating?
Maybe they would be ok using property overloading?

[mmiller-max]

Yeah make sense, happy to leave for now. I've got a possible use case for it, but also didn't appreciate that you could do property overloading.