rework unit tests (mocking the fetch())

[ftoromanoff]

This PR propose a rewrite of the unit tests:

• minor corrections in tests
• mocking of the 'fetch()' function to have unit tests to run independent from network
  • to do that the files that should have been fetch have been added localy.
  • we use sinon to mock itowns.Fetcher
• add a file test for testing Fetcher.js

Modifications:

• add 'sinon' library to do the mocking
• all tests
  • add all test files in local folder test/data/unitTest
  • change global url to local ones
• add fetcher.js to test Fetcher.js
  • add library xml dom for xml fetcher

[jailln]

I think we could factorize a bit this mocking logic in one place (e.g. in utils.js) by creating the following method:

function getFetcherStub(method, returnVal) {
    sinon.stub(Fetcher, method)
    .callsFake(() => Promise.resolve(returnVal));
}

And just do stubFetcherJson = utils.getFetcherStub('json', tileset) here (and use this method everywhere else where you create stubs). WDYT?

[ftoromanoff]

The purpose of this PR is to get in the direction of real unti tests. Thus i did mock the fetching everywhere, but later on we should be mocking the process that use the mocked fetcher (except for some specifique tests).That's why I think that a it will be on some further PR to think about how to factorize the different mocking parts.

[jailln]

Ok, in this case could you open an issue about it please ?