Added a new unit test 'test/server.cpp'. The pre-existing unit test
test/kiwixserve.cpp tests kiwixlib indirectly by running the kiwix-serve
executable which may be built with another version of the library.
Included with this commit is the cpp-httplib C++ HTTP/HTTPS library
(https://github.com/yhirose/cpp-httplib, v0.5.11).
The new file test/httplib.h was copied from
https://raw.githubusercontent.com/yhirose/cpp-httplib/v0.5.11/httplib.h
Veloman Yunkan