use Google C++ Testing Framework for libapt tests

use Google C++ Testing Framework for libapt tests

My commit 45df0ad2 from 26. Nov 2009 had a little remark:
"The commit also includes a very very simple testapp."
This was never intended to be permanent, but as usually…

The commit adds the needed make magic to compile gtest statically
as it is required and links it against a small runner. All previous
testcase binaries are reimplemented in gtest and combined in this
runner. While most code is a 1:1 translation some had to be rewritten
like compareversion_test.cc, but the coverage remains the same.