[ticket/11549] Do not set extension dir path for style in adm/index.php

As we don't want style files in adm/adm/style/ we should not prepend the adm/
by setting it. Note, we did that correctly in adm/swatch.php.

PHPBB3-11549
[ticket/11549] Add functional test for ACP Extension Module with Template

PHPBB3-11549
Merge remote-tracking branch 'nickvergessen/ticket/11549' into develop

* nickvergessen/ticket/11549:
  [ticket/11549] Add functional test for ACP Extension Module with Template
  [ticket/11549] Do not set extension dir path for style in adm/index.php