Make code-assert a test dependency

[sirocchj]

code-assert is using checkstyle:6.19 that pulls in commons-beanutils:1.9.2, which is subject to CVE-2019-10086 (see also here).

While this addresses the issue in code-assert, I believe this dependency should not be scoped to compile in graphviz-java to begin with, unless I am missing something.

[codecov-io]

Codecov Report

Merging #198 (a0eeb74) into master (f0c1fdf) will decrease coverage by 0.15%. The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #198      +/-   ##
============================================
- Coverage     83.81%   83.66%   -0.16%     
+ Complexity     1279     1277       -2     
============================================
  Files           109      109              
  Lines          3813     3813              
  Branches        385      385              
============================================
- Hits           3196     3190       -6     
- Misses          402      408       +6     
  Partials        215      215              

Impacted Files	Coverage Δ	Complexity Δ
...n/java/guru/nidi/graphviz/service/SystemUtils.java	50.00% <0.00%> (-13.16%)	13.00% <0.00%> (-2.00%)
...ru/nidi/graphviz/engine/GraphvizCmdLineEngine.java	86.40% <0.00%> (-0.98%)	30.00% <0.00%> (ø%)

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[sirocchj]

Hey @nidi3 what do you think about this? Does it make sense to you?