Cleanup phpunit.xml.dist

We usually follow the rule that comment statement should be above the code which it belongs to. The php section of the phpunit.xml.dist does not comply with this.

<php>
  <!-- Set error reporting to E_ALL. -->
  <ini name="error_reporting" value="32767"/>
  <!-- Do not limit the amount of memory tests take to run. -->
  <ini name="memory_limit" value="-1"/>
  <env name="SIMPLETEST_BASE_URL" value=""/>
  <!-- Example SIMPLETEST_BASE_URL value: http://localhost -->
  <env name="SIMPLETEST_DB" value=""/>
  <!-- Example SIMPLETEST_DB value: mysql://username:password@localhost/databasename#table_prefix -->
  <env name="BROWSERTEST_OUTPUT_DIRECTORY" value=""/>
  <!-- Example BROWSERTEST_OUTPUT_DIRECTORY value: /path/to/webroot/sites/simpletest/browser_output -->
</php>