Implement comprehensive tests

This module is relatively simple, but it doesn't mean we should not implement comprehensive tests for it.

Here is my preliminary list:

Test the autocomplete widget

For every core type, add a entityreference to a node type (Widget: autocomplete, Display: label)
Visit the node add page
Submit the autocomplete
Verify on the page that the label of the entity is displayed

Test the select widget

For every core type, add a entityreference to a node type (Widget: select, Display: label)
Visit the node add page
Verify the option list
Submit
Verify on the page that the label of the entity is displayed

Test the label display

Add a entityreference to a node type
Create a node referencing itself

Asserts:
- the label is displayed
- the link is displayed in link mode

Test the build mode display

Add a entityreference to a node type
Create a node referencing itself

Asserts:
- the class of the build mode is displayed
- the title of the node is displayed inside
- the expected fields are displayed

Test the Views integration