Update unit-tests.dox to replace reference to oldkea.isc.org

Update unit-tests.dox to replace reference to oldkea.isc.org

@section unitTestsIntroduction Introduction

Kea uses the Google C++ Testing Framework (also called googletest or gtest) as a
base for our C++ unit-tests. See https://github.com/google/googletest for
details. We used to have Python unit-tests inherited from BIND10
days but have been removed, so please do not write any new Kea unit
tests in Python. If you want to write DHCP tests in Python, we encourage you to
take a look at ISC Forge: http://oldkea.isc.org/wiki/IscForge .

You must have \c gtest installed or at least extracted in a directory
before compiling Kea unit-tests.  To enable unit-tests in Kea, use:

@code
./configure --with-gtest=/path/to/your/gtest/dir
@endcode

or

@code
./configure --with-gtest-source=/path/to/your/gtest/dir