[ticket/11538] Add unit tests for phpbb_validate_hex_colour().

PHPBB3-11538
[ticket/11538] Compare to '' instead of using empty(), so '0' does not pass.

PHPBB3-11538
[ticket/11538] Cut functional tests down a bit as these are more expensive.

PHPBB3-11538
Merge remote-tracking branch 'remotes/bantu/ticket/11538' into develop-olympus

* remotes/bantu/ticket/11538:
  [ticket/11538] Cut functional tests down a bit as these are more expensive.
  [ticket/11538] Compare to '' instead of using empty(), so '0' does not pass.
  [ticket/11538] Add unit tests for phpbb_validate_hex_colour().