This commit adds test helper functions to drag and drop
files:
- `createFile`: create a file that can be used for drag and
drop in tests.
- `dragoverFile`: to drag a file created from `createFile`
over a DOM element.
- `dropFile`: to drop a file created from `createFile` on
a DOM element.
These utility functions are available from `testUtils.file`.
Example of usage:
```js
var testUtils = require('web.test_utils');
testUtils.file.createFile({
name: 'text.txt',
content: 'hello, world',
contentType: 'text/plain',
}).then(function (file) {
testUtils.file.dragoverFile($el, file);
testUtils.file.dropFile($el, file);
});
```
Xavier Dubuc