[WIP][Testing] Fix phpunit test dir paths

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Maybe there could be further improvements: The paragraph that follows is a bit confusing. It says *...the following configuration adds the tests from the installed third-party bundles* and then the tests from AcmeDemoBundle are included with `<directory>../src/Acme/Bundle/*Bundle/Tests</directory>`. Mh, the AcmeDemoBundle is a 3rd party bundle that resides in `src`? Shouldn't it include a directory from `vendors` instead to demonstrate this? In addition, the AcmeDemoBundle tests are covered by the standard configuration, so there should be no need for an extra configuration.

It continues with *To include other directories in the code coverage..* and then some dirs from the AcmeDemoBundle are excluded which is also covered by the standard configuration.

What do you think, should I make some more updates here?