Fix kdepim-addons autotest EnterpriseHeaderStylePluginTest

Summary:
EnterpriseHeaderStyle::format() returns a chunk of HTML ending with an
unclosed DIV.  This patch adds a "</div>" tag in the tests, so that
testHeaderFile() can construct valid XHTML to pass to xmllint, and adds
the DIV to the expected output.

See also D22892.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: dvratil, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D23252