test: fix flaky tests

[ylemkimon]

• 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?

N/A

Motivation / Use-Case

• Some tests were using the default port 8080, causing EADDRINUSE on collision. Also, added a warning to prevent future mistakes.
  • https://github.com/webpack/webpack-dev-server/runs/1413577706
• Some proxy-option tests were ended before all servers were closed (not sure this would cause failures)

Breaking Changes

N/A

Additional Info

[codecov]

Codecov Report

Merging #2842 (64c9d9f) into v4 (e6f91ad) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##               v4    #2842   +/-   ##
=======================================
  Coverage   93.08%   93.08%           
=======================================
  Files          40       40           
  Lines        1331     1331           
  Branches      354      354           
=======================================
  Hits         1239     1239           
  Misses         88       88           
  Partials        4        4           

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[alexander-akait]

Thanks