Improve a few unit tests and classes

[ham1]

Description

• Formatting of SampleComparator and FieldSampleComparator
• Moved TestDNSCacheManager to Spock
• Formatted and refactored some parts of DNSCacheManager
• Added more tests for FieldSampleComparatorSpec

How Has This Been Tested?

ant clean test-headless -Drmi_force_localhost=true

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (trunk@be3271f). Click here to learn what that means.
The diff coverage is 83.1%.

@@           Coverage Diff            @@
##             trunk     #451   +/-   ##
========================================
  Coverage         ?   58.71%           
  Complexity       ?    10818           
========================================
  Files            ?     1209           
  Lines            ?    77304           
  Branches         ?     7466           
========================================
  Hits             ?    45389           
  Misses           ?    29373           
  Partials         ?     2542

Impacted Files	Coverage Δ	Complexity Δ
.../report/processor/FieldSampleComparatorSpec.groovy	100% <100%> (ø)	4 <3> (?)
...jmeter/report/processor/FieldSampleComparator.java	100% <100%> (ø)	3 <1> (?)
.../jmeter/protocol/http/control/DNSCacheManager.java	74.05% <60%> (ø)	37 <6> (?)
...r/protocol/http/control/DNSCacheManagerSpec.groovy	91.35% <91.35%> (ø)	13 <13> (?)

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