Simplify visualizer tests

[jawira]

In order to avoid errors in VisualizerTask tests, I removed any reference to any external PlantUML server.

[codecov]

Codecov Report

Merging #1247 into master will decrease coverage by 0.15%.
The diff coverage is 59.51%.

@@             Coverage Diff             @@
##             master   #1247      +/-   ##
===========================================
- Coverage     49.65%   49.5%   -0.16%     
  Complexity     9631    9631              
===========================================
  Files           492     492              
  Lines         23568   23568              
===========================================
- Hits          11703   11667      -36     
- Misses        11865   11901      +36

Impacted Files	Coverage Δ	Complexity Δ
classes/phing/tasks/ext/XmlPropertyTask.php	0% <ø> (ø)	12 <0> (ø)	⬇️
...ses/phing/tasks/ext/phpunit/PHPUnitTestRunner8.php	0% <0%> (ø)	58 <0> (ø)	⬇️
classes/phing/types/selectors/SelectorAware.php	28.07% <0%> (ø)	28 <1> (ø)	⬇️
classes/phing/tasks/ext/svn/SvnBaseTask.php	0% <0%> (ø)	48 <2> (ø)	⬇️
classes/phing/Diagnostics.php	0% <0%> (ø)	19 <0> (ø)	⬇️
classes/phing/listener/MonologListener.php	10.2% <10.2%> (ø)	17 <17> (ø)	⬇️
classes/phing/RuntimeConfigurable.php	94.59% <100%> (ø)	20 <1> (ø)	⬇️
classes/phing/IntrospectionHelper.php	67.11% <100%> (ø)	85 <0> (ø)	⬇️
...s/phing/tasks/system/condition/IsTrueCondition.php	83.33% <100%> (ø)	3 <1> (ø)	⬇️
classes/phing/tasks/system/DynamicTask.php	100% <100%> (ø)	2 <2> (ø)	⬇️
... and 36 more

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[siad007]

@jawira thx