refactor(test): move test assets to tests dir

[mastermatt]

Also, rename fixture files to use consistent snake_case.

[paulmelnikow]

Can you make sure npm pack isn’t catching these files post move?

[mastermatt]

0 ~/src/nock: npm pack --dry-run
npm notice
npm notice 📦  nock@0.0.0-development
npm notice === Tarball Contents ===
npm notice 1.1kB  LICENSE
npm notice 6.8kB  lib/back.js
npm notice 19.6kB lib/common.js
npm notice 1.2kB  lib/delayed_body.js
npm notice 94B    lib/global_emitter.js
npm notice 1.3kB  index.js
npm notice 13.6kB lib/intercept.js
npm notice 9.5kB  lib/intercepted_request_router.js
npm notice 17.9kB lib/interceptor.js
npm notice 2.1kB  lib/match_body.js
npm notice 10.6kB lib/playback_interceptor.js
npm notice 12.1kB lib/recorder.js
npm notice 10.7kB lib/scope.js
npm notice 1.8kB  lib/socket.js
npm notice 2.4kB  package.json
npm notice 353B   CHANGELOG.md
npm notice 51.4kB README.md
npm notice 7.7kB  types/index.d.ts
npm notice === Tarball Details ===
npm notice name:          nock
npm notice version:       0.0.0-development
npm notice filename:      nock-0.0.0-development.tgz
npm notice package size:  48.7 kB
npm notice unpacked size: 170.0 kB
npm notice shasum:        9f50eede3cec6f0568d54d2d943d4b254deb5eba
npm notice integrity:     sha512-piGFypiADC15A[...]0Wwwt5ngYm+Cw==
npm notice total files:   18

[github-actions]

🎉 This PR is included in version 12.0.2 🎉

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