Exclude directories - Use AbsolutePath

When you run phpunit using a configuration file where the current working directory does not match the configuration's directory, phpunit will fail to exclude paths in testsuites correctly.

Sample configuration:

```<testsuites>
  <testsuite name="Normal Test Suite">
    <directory>.</directory>
    <exclude>LongTests</exclude>
    <exclude>FlakyTests</exclude>
  </testsuite>
```

If you run that configuration from any other directory, the <directory>.</directory> will go to the absolute path of the configuration, while the excludes will only exclude from your current working directory (which won't match the . directory).