[ADD] web:tests: test utils for drag/drop files

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);
});
```

0 files changed, 0 insertions, 0 deletions