Introducing 1st real unit test of the server

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

test/meson.build

@@ -11,7 +11,8 @@ tests = [
     'pathTools',
     'kiwixserve',
     'book',
-    'manager'
+    'manager',
+    'server'
 ]
 
 
@@ -21,8 +22,15 @@ gtest_dep = dependency('gtest',
                        required:false)
 
 if gtest_dep.found() and not meson.is_cross_build()
+    configure_file(input : 'data/wikipedia_en_ray_charles_mini_2020-03.zim',
+                   output : 'zimfile.zim',
+                   copy: true )
+
     foreach test_name : tests
+        # XXX: implicit_include_directories must be set to false, otherwise
+        # XXX: '#include <regex>' includes the regex unit test binary
         test_exe = executable(test_name, [test_name+'.cpp'],
+                              implicit_include_directories: false,
                               link_with : kiwixlib,
                               link_args: extra_link_args,
                               dependencies : all_deps + [gtest_dep],