test(Util, Validation): close server each time test ends

[hiroppy]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

no

Motivation / Use-Case

resolve ConcurrentCompilationError: You ran Webpack twice. Each instance only supports a single concurrent compilation at a time.

Breaking Changes

Additional Info

via #1673

[codecov]

Codecov Report

Merging #1680 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1680      +/-   ##
==========================================
+ Coverage   76.17%   76.34%   +0.16%     
==========================================
  Files          17       17              
  Lines         596      596              
  Branches      173      173              
==========================================
+ Hits          454      455       +1     
+ Misses        108      107       -1     
  Partials       34       34

Impacted Files	Coverage Δ
lib/Server.js	80.15% <0%> (+0.26%)	⬆️

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[alexander-akait]

Awesome job!