test: unit tests for server module

[clarkdo]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[codecov-io]

Codecov Report

Merging #5154 into dev will increase coverage by 0.54%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##              dev   #5154      +/-   ##
=========================================
+ Coverage   96.06%   96.6%   +0.54%     
=========================================
  Files          73      72       -1     
  Lines        2463    2447      -16     
  Branches      625     620       -5     
=========================================
- Hits         2366    2364       -2     
+ Misses         81      68      -13     
+ Partials       16      15       -1

Impacted Files	Coverage Δ
packages/server/src/middleware/nuxt.js	100% <ø> (+1.47%)	⬆️
packages/server/src/middleware/error.js	100% <100%> (+2.12%)	⬆️
packages/vue-renderer/src/renderer.js	95.85% <0%> (+1.03%)	⬆️
packages/server/src/listener.js	100% <0%> (+1.88%)	⬆️
packages/server/src/middleware/modern.js	100% <0%> (+3.12%)	⬆️
packages/server/src/middleware/timing.js	100% <0%> (+4.54%)	⬆️
packages/server/src/server.js	100% <0%> (+5.74%)	⬆️

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