WUT: add testkit folder to `includes` of `tsconfig` (#6998)
WUT: add testkit folder to `includes` of `tsconfig` (#6998)

`src/testkit` folder contains testkit exports files for multiple
testing platforms. All those files are considered entry points for users
and so all of them should end up in the released package.

However, only those files that are required by other files ended up in
`dist/src/testkit`.
for example, because some `.spec.js` or `.e2e.js` require
`src/testkit/enzyme.js` or `src/testkit/protractor`, these files end up
properly in `dist/src/testkit`.

`src/testkit/puppeteer.ts`, however, does not. It was not used by any
other file and `tsconfig.json` `includes` array does not have a glob
matching that.
wix/wix-style-react,wix/wix-style-react,wix/wix-style-react,wix/wix-style-react