Re-organise folders structure

To follow best practices and allow us to have a better ability to test and develop new features, I would like to open discussion around packages reorganisation

toxiproxy/
├── bin/
├── cmd/
│   ├── cli/
│   │   └── main.go
│   ├── server/
│   │   └── main.go
│   └── dashboard
│       └── main.go
├── pkg/
│   ├── client/
│   │   └── ...
│   ├── server/
│   │   └── ...
│   ├── dashboard/
│   │   └── ...
│   └── toxics/
│   │   └── ...
│   └── stream/
│   │   └── ...
│   └── e2e/ (renamed `testing` see below)
│   │   └── ...
│   └── toxiproxytest/ (renamed `testhelper` see: https://pkg.go.dev/net/http/httptest)
│   │   └── ...
├── ...
└── README.md

Also I think it makes sense to restructure testing folder, since one of the files endpoint is used in Makefile to run e2e (maybe we just should rename it to be e2e/main.go) and benchmark_test.go is used only for benchmarking also in e2e tests.

Review the proposed cmd/ and pkg/ layout alongside the existing testing folder and Makefile. Start by tracing how endpoint, benchmark_test.go, and the e2e tests are invoked. Done means the package boundaries and naming are agreed and the reorganized tests and Makefile commands still work.