Minor fixes at tests

[ronanrodrigo]

• Update LinuxMain.swift
• Remove unnecessary setup method at URLRequestFactoryTests
• Extract test structs to files
• Extract XCTest extension to file
• Rename some tests

[codecov-io]

Codecov Report

Merging #13 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   95.03%   95.03%           
=======================================
  Files           8        8           
  Lines         141      141           
  Branches       11       11           
=======================================
  Hits          134      134           
  Misses          7        7

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