Improve Unit tests readability and use of Spock

[ham1]

Description

Improve the readability and reduced complexity of many Spock tests.

Sorry for the large-ish PR but I've tried to make the commits reasonably small and distinct.

How Has This Been Tested?

ant test-headless -Drmi_force_localhost=true

[codecov-io]

Codecov Report

Merging #446 into trunk will increase coverage by <.01%.
The diff coverage is 91.66%.

@@             Coverage Diff              @@
##              trunk     #446      +/-   ##
============================================
+ Coverage     58.58%   58.59%   +<.01%     
- Complexity    10786    10787       +1     
============================================
  Files          1206     1206              
  Lines         77258    77253       -5     
  Branches       7464     7460       -4     
============================================
- Hits          45264    45263       -1     
  Misses        29459    29459              
+ Partials       2535     2531       -4

Impacted Files	Coverage Δ	Complexity Δ
...c/org/apache/jmeter/engine/util/PackageSpec.groovy	97.43% <ø> (ø)	6 <0> (ø)	⬇️
...pache/jmeter/assertions/MD5HexAssertionSpec.groovy	78.26% <ø> (ø)	6 <0> (ø)	⬇️
...t/processor/Top5ErrorsBySamplerConsumerSpec.groovy	93.1% <ø> (ø)	5 <0> (ø)	⬇️
...rc/org/apache/jmeter/junit/spock/JMeterSpec.groovy	9.09% <ø> (ø)	3 <0> (ø)	⬇️
...meter/protocol/jdbc/sampler/JDBCSamplerSpec.groovy	100% <ø> (ø)	7 <0> (ø)	⬇️
...c/org/apache/jmeter/services/FileServerSpec.groovy	91.3% <0%> (ø)	21 <0> (ø)	⬇️
...che/jmeter/control/ThroughputControllerSpec.groovy	96.19% <100%> (ø)	11 <2> (ø)	⬇️
.../report/processor/FieldSampleComparatorSpec.groovy	100% <100%> (ø)	2 <2> (?)
...t/src/org/apache/jorphan/util/ConverterSpec.groovy	100% <100%> (ø)	7 <6> (ø)	⬇️
.../report/processor/Top5ErrorsSummaryDataSpec.groovy	100% <100%> (ø)	6 <3> (ø)	⬇️
... and 7 more

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