Name already in use

We have to remove `--failAfterWarnings` from our Rollup integration-test,
because Typescript, when targeting es5, will use global `this` for transpilation.
Rollup warns about this, since it replaces `this` with `undefined`:

```
src/index.js → dist/bundle.js...
(!) `this` has been rewritten to `undefined`
https://rollupjs.org/guide/en/#error-this-is-undefined
../../../node_modules/@handlebars/parser/dist/esm/printer.js
1: var __spreadArrays = (this && this.__spreadArrays) || function () {}
```

See https://github.com/handlebars-lang/handlebars-parser/releases/tag/v2.0.0