Add spy tests to core

[ksaaskil]

• Copied spy-vs-spy.test.ts from unmock-node to unmock-core
• Fixed a bug in unmock-fetch where init was not passed to serializer
• [ ] Some of the spy tests in unmock-node could not be yet ported as fetch serializer does not serialize headers or body

[codecov-io]

Codecov Report

Merging #332 into dev will increase coverage by 7.12%. The diff coverage is 100%.

@@            Coverage Diff            @@
##              dev    #332      +/-   ##
=========================================
+ Coverage   72.67%   79.8%   +7.12%     
=========================================
  Files          51      51              
  Lines        2174    2174              
  Branches      563     563              
=========================================
+ Hits         1580    1735     +155     
+ Misses        594     439     -155

Impacted Files	Coverage Δ
packages/unmock-fetch/src/fetch.ts	92% <100%> (ø)	:arrow_up:
packages/unmock-core/src/generator.ts	87.59% <0%> (+1.87%)	:arrow_up:
...ges/unmock-core/src/service/spy/generated/index.ts	69.49% <0%> (+50.84%)	:arrow_up:

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