[ticket/11537] Adjust error message on ucp group manage to fit rest of ucp

The current look highly differed from the rest of the UCP. Additionally,
this caused the error message to be hardly noticeable at all.

PHPBB3-11537
[ticket/11538] Make sure group color can't exceed maximum of 6 characters

In order to prevent future issues with this, a basic set of functional
tests for the UCP groups manage page have been added.

PHPBB3-11538
[ticket/11540] Add unit tests for is_absolute()

PHPBB3-11540
[ticket/11538] Use regex for testing color value and improve tests

We are now using a regex with preg_match() in order to properly check
if the entered color value is in hex color format or not. A proper
error message is triggered if an incorrect color value is entered and
the prepended '#' is removed if necessary.

PHPBB3-11538
[ticket/11541] Add unit tests for style_select()

PHPBB3-11541
[ticket/11542] Add unit tests for language_select()

PHPBB3-11542
[ticket/11538] Fix incorrect regex and test for duplicate # in color string

PHPBB3-11538
[ticket/11538] Simplify colour value check and remove support for '#'

The input length for the hex color is now limited to 6 characters and
the support for colors starting with a '#' has been dropped. The allowed
input length of 7 in prosilver seems to have been a relict from old ages
of phpBB3. In order to have proper support for correct checking of the
colour value, the new code was also ported to the ACP groups manage page.
The tests have been modified to reflect the changes to the behavior of
the color check. Tests for the ACP will follow.

PHPBB3-11538
[ticket/11538] Add tests for acp group manage page

This will currently test if the colour check properly works.

PHPBB3-11538
[ticket/11544] Add admin_login() method to 3.0 functional test case

This method is needed in order to be able to properly test acp functions.

PHPBB3-11544
[ticket/11538] Make sure regex doesn't allow multiple color values

This will now make sure that 'AAFF00' will be matched but strings like
'AAFF00 AABB00' will not.

PHPBB3-11538
Merge remote-tracking branch 'nickvergessen/ticket/11540' into develop-olympus

* nickvergessen/ticket/11540:
  [ticket/11540] Add unit tests for is_absolute()
[ticket/11546] Fix is_absolute() throws E_NOTICE for empty string

PHPBB3-11546
Merge remote-tracking branch 'nickvergessen/ticket/11546' into develop-olympus

* nickvergessen/ticket/11546:
  [ticket/11546] Fix is_absolute() throws E_NOTICE for empty string
Merge remote-tracking branch 'nickvergessen/ticket/11541' into develop-olympus

* nickvergessen/ticket/11541:
  [ticket/11541] Add unit tests for style_select()
[ticket/11536] Fixed incorrect removal of "install" in script_path

Earlier the script would incorrectly remove the word "install"
from the script_path when trying to remove the last folder in the
path named "/install". This would lead to issues when the path
you are installing phpBB contained "install". For example
"/install_test/install" would become "/_test".

This change gets the parent folder instead of replacing all
"install". $name contains /install_test/install/index.php
from start and running dirname() gives /install_test/install.
Running dirname once more gives the parent folder of install,
/install_test.

PHPBB3-11536
[ticket/11542] Add lang_english_name to fixture

PHPBB3-11542
[ticket/11538] Add function phpbb_validate_colour for validating colours

It will be possible to use this function via the validate_data() function
interface that has already been used previously. Thus, this new function
will extend the capabilities of validate_data() to checking hex color
values.

PHPBB3-11538
[ticket/11545] Remove DIRECTORY_SEPARATOR dependency from is_absolute

The given path is an absolute path in general, just not on our current system.

PHPBB3-11545
Merge remote-tracking branch 'nickvergessen/ticket/11545' into develop-olympus

* nickvergessen/ticket/11545:
  [ticket/11545] Remove DIRECTORY_SEPARATOR dependency from is_absolute