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?