Split test/server.cpp

The file starts now to be too long.

- Move testing of the search html result in `test/server_html_search.cpp`
- Move common code used to launch server and so
  in `test/server_testing_tools.h'

This is mainly code move with a small change:
Instead of setting the default PORT (8001) as a const int in the
`ServerTest` class, we now use SERVER_PORT.
SERVER_PORT must be defined before include `server_testing_tools.h`.
This allow several test to be run in parallele without trying to open
the same port.

test/meson.build

@@ -15,7 +15,10 @@ tests = [
 ]
 
 if build_machine.system() != 'windows'
-  tests += ['server']
+  tests += [
+      'server',
+      'server_html_search'
+  ]
 endif