[feat] Add option to run student test classes instead of reference tests

[slarse]

Fix #53

[codecov]

Codecov Report

Merging #57 into master will decrease coverage by 0.78%.
The diff coverage is 90.38%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
- Coverage   94.36%   93.57%   -0.79%     
==========================================
  Files           5        6       +1     
  Lines         284      327      +43     
  Branches       35       47      +12     
==========================================
+ Hits          268      306      +38     
- Misses         10       13       +3     
- Partials        6        8       +2

Impacted Files	Coverage Δ
repobee_junit4/junit4.py	95.62% <100%> (+0.13%)	⬆️
repobee_junit4/_exception.py	100% <100%> (ø)
repobee_junit4/_java.py	90.74% <83.33%> (-2.85%)	⬇️

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