Add more tests

[mhauru]

Something I noticed while writing these tests: getindex for MetricProblem returns a Problem rather than a MetricProblem:

Base.getindex(p::MetricProblem{Vector{IOExample}}, indices) = Problem(p.spec[indices])

Is this intentional?

[mhauru]

@pwochner

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d9de26b) to head (a3e2bc9).
Report is 8 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##           master       #13       +/-   ##
============================================
+ Coverage   50.00%   100.00%   +50.00%     
============================================
  Files           2         1        -1     
  Lines          12        14        +2     
============================================
+ Hits            6        14        +8     
+ Misses          6         0        -6     

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[THinnerichs]

Good catch, this should indeed return a MetricProblem. We currently do not have any metric problem within our benchmarks at this point. Hence, this did not come up earlier.

[ReubenJ]

Looks great, thanks!