Allow and test for custom TestSets in pkg tests

[mmiller-max]

The changes here mean that if a custom TestSet has any of the fields properties, time_taken, start_time or hostname then they are used in the report. This could break/be strange if these fields are in the test set but used for other things and not what we use them for. Is it better to only populate these values in a report when ReportingTestSets are used or to keep the flexibility for custom TestSets?

Closes #61

[oxinabox]

I wonder if rather than checking e.g. hasproperty(ts, :time_taken)....
we should just instead do:

time_taken(ts::ReportingTestSet) = ts.time_taken
time_taken(ts::AbstractTestSet) = nothing  # or some dummy value like -1ms

and then it is easy enough to overload that to add support for them

[mmiller-max]

Yeah that's better, probably worth doing for the description and results too, although #3 mentions that these could be in base

[oxinabox]

Adding things in Base takes ages, so probably not worth it.

[codecov-io]

Codecov Report

Merging #63 (410234a) into master (c4ecb80) will decrease coverage by 1.36%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   92.00%   90.63%   -1.37%     
==========================================
  Files           5        6       +1     
  Lines         375      395      +20     
==========================================
+ Hits          345      358      +13     
- Misses         30       37       +7     

Impacted Files	Coverage Δ
src/TestReports.jl	100.00% <ø> (ø)
src/v1_compat.jl	0.00% <0.00%> (ø)
src/testsets.jl	95.04% <80.00%> (-4.96%)	⬇️
src/recordproperty.jl	100.00% <100.00%> (ø)
src/to_xml.jl	100.00% <100.00%> (ø)

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[mmiller-max]

OK I've updated it following your suggestions. I've left results and properties for now as it was more involved (we set both of these as we flatten TestSets, for example. I'll add some notes to #3.

If you're happy with this, I'll add some more docs and tests to make sure its all being properly tested