Re-organise folders structure

[Strech]

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.

[Strech]

Another way to rearrange files around testing is to rename bin folder into test and move there testing folder too (inspiration https://github.com/kubernetes-sigs/descheduler/tree/master/test)

=== before

toxiptoxy/
├── bin/
│   ├── e2e
│   └── release_test
├── testing/
│   ├── benchmark_test.go
│   └── endpoint.go
...

=== after

toxiptoxy/
├── test/
│   ├── e2e/
│   │   ├── benchmark_test.go
│   │   └── endpoint.go
│   ├── run-e2e-tests
│   └── run-release-tests
...

[Strech]

Agreed structure for testing

toxiptoxy/
├── scripts/
│   ├── test-e2e
│   └── test-release
├── test/
│   ├── e2e/
│   │   ├── benchmark_test.go
│   │   └── endpoint.go
...