Make emit work in jest

[paolochiodi]

This should fix #2, which is also similar to other issues reported to fastify itself (i.e. see fastify/fastify#2438)

The underlying cause here is jest fiddling with sandboxing, context and globals. The result is that the Error global object available to fastify-warning is not the same object in the main node process and thus FastifyWarning instanceOf Error that is run in the implementation of process.emitWarning (see https://github.com/nodejs/node/blob/v10.22.0/lib/internal/process/warning.js#L146) fails.

The issues caused by jest sandboxing model have been known for a while now (see jestjs/jest#2549 and jestjs/jest#8246) and looks like they won't be solved soon.

This PR works around it by not emitting an error, but rather calling emitWarning with string params instead (documented here: https://nodejs.org/dist/latest-v12.x/docs/api/process.html#process_process_emitwarning_warning_type_code_ctor)

The downside of this approach is that warning.create won't be returning a constructor for an object that inherits from Error, but that apparently was never used in fastify.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows Code of conduct

[mcollina]

lgtm

[delvedor]

LGTM

[delvedor]

Landed in v0.2.0, thanks!