Fix testing submodule build and export

[cuzzlor]

A couple of issues with v1.4.0: the addition of the /testing submodule:

1. ./src/testing/index.ts was missing from the rollup config so the code was not included (only the type defs)
2. ./testing was missing from the package.json exports section

Fixing the exports results in this generated package.json content:

  "main": "./index.js",
  "module": "./index.mjs",
  "types": "./index.d.ts",
  "type": "commonjs",
  "exports": {
    ".": {
      "types": "./index.d.ts",
      "import": "./index.mjs",
      "require": "./index.js"
    },
    "./testing": {
      "types": "./testing/index.d.ts",
      "import": "./testing/index.mjs",
      "require": "./testing/index.js"
    }
  }

[mderriey]

Fixing the exports results in this generated package.json content

👌