EZP-21422: Fixed failing PHPUnit tests because of unsupported assertType()

`PHPUnit_Framework_TestCase::assertType()` has been removed from current PHPUnit distributions, causing the PHPUnit tests for this extension to fail.

With this pull request, the following changes are made:
- Replaced `assertType( 'string|int|array' )` with `assertTrue( is_string()|is_int()|is_array )`
- Replaced `self::assert*` with `$this->assert*` because it is common style
- Added a missing parameter to the PHPDoc of `testFieldsLinksByContent()`

Cheers!
:octocat: Jérôme