test: assert 3rd party reexports

[antongolub]

zx reexports some 3rd party libs, but when their API changes it may bring unexpected breaking changes. Adding and maintaining full test suites for yaml, fs-extra, minimist, globby, etc it will be too expensive for us, but let's at least inspect the helpers contents:

Seems possible to autogenerate smth like this
test/vendor-yaml.test.js

// ...
import {yaml} from 'zx'

describe('vendor yaml', () => {
   it('has proper exports', () => {
     assert.equal(typeof yaml.parse, 'function')
     assert.equal(typeof yaml.stringify, 'function')
     assert.equal(typeof yaml.parseAllDocuments, 'function')
     // ...
   })
})