Refactor Java-related code

[slarse]

Fix #20

[codecov]

Codecov Report

Merging #21 into master will decrease coverage by 0.78%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
- Coverage   96.55%   95.76%   -0.79%     
==========================================
  Files           3        5       +2     
  Lines         203      236      +33     
==========================================
+ Hits          196      226      +30     
- Misses          7       10       +3

Impacted Files	Coverage Δ
repomate_junit4/__version.py	100% <100%> (ø)	⬆️
repomate_junit4/__init__.py	100% <100%> (ø)	⬆️
repomate_junit4/junit4.py	96.49% <86.66%> (-0.03%)	⬇️
repomate_junit4/_java.py	94.87% <94.87%> (ø)
repomate_junit4/_junit4_runner.py	95.12% <95.12%> (ø)

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