test: achieve 100% code coverage

[darkgl0w]

Hello,

• reaching 100% code coverage (fix 100% code coverage #112)
• remove superfluous and dead code
• add adm-zip as a new dev-dep in order to test a specific use case
• remove the check preventing multiple onSend hook triggers as it was fixed in fastify upstream (possible regressions are monitored by a test case)

This PR needs a little bit of work to be completed but early reviews are welcome ^^
EDIT: it is now ready to be reviewed

Note : Tests and files should be reorganized and structured but I will do this in a new PR (reviewing will be a pain otherwise).
To be done :

• Reorganize test files (renaming following xxx.test.js convention)
• Reorganize tests (making groups with sub-tests)
• Maybe going full async/await syntax test path
• Improve typescript types and tests (should investigate a bit more about what can be done)
• Improve the documentation

main branch :

  🌈 SUMMARY RESULTS 🌈  

Suites:   ​4 passed​, ​4 of 4 completed​
Asserts:  ​​​341 passed​, ​of 341​
​Time:​   ​9s​
----------|---------|----------|---------|---------|---------------------------------------------------------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s                                                   
----------|---------|----------|---------|---------|---------------------------------------------------------------------
All files |   92.01 |    83.41 |   89.18 |    93.2 |                                                                     
 index.js |   92.01 |    83.41 |   89.18 |    93.2 | 127,129,137,169,189,197,199,215,217,282,284,371-374,484-485,510-511 
----------|---------|----------|---------|---------|---------------------------------------------------------------------

after :

  🌈 SUMMARY RESULTS 🌈
​
Suites:   ​4 passed​, ​4 of 4 completed​
Asserts:  444 passed​, ​of 444​
​Time:​   ​10s​
----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |     100 |      100 |     100 |     100 |                   
 index.js |     100 |      100 |     100 |     100 |
----------|---------|----------|---------|---------|-------------------

Checklist

• run npm run test
• tests are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

good work!

[darkgl0w]

The PR is ready for reviews.

[mcollina]

Thanks, this is amazing!

[mcollina]

lgtm