Doxyfile: Suppress warnings for phpunit "@" annotations

Doxyfile: Suppress warnings for phpunit "@" annotations

This annotations are not relevant for Doxygen.

Examples:
> parserTest.inc:320: warning: Found unknown command `\group'
> ActionTest.php:3: warning: Found unknown command `\covers'
> ActionTest.php:128: warning: Found unknown command `\dataProvider'
> ApiBlockTest.php:67: warning: Found unknown command `\expectedException'
> ApiBlockTest.php:68: warning: Found unknown command `\expectedExceptionMessage'

Change-Id: I1d2d617ee6c15d51943bec3563ea4ffd353baec4