1) Remove the non-Windows-only condition for subdir('tests').
2) Add libiphlpapi, libws2_32 and libsecur32 deps, needed for W32 tests.
3) Remove the -no-undefined argument (gcc doesn't understand it,
   it *does* understand -Wl,-no-undefined; either way, the test
   compiles without this argument just fine; maybe meson adds it
   by itself - you can hardly build shared modules without it).
4) Add or fix a number of includes
5) Disable gdbus-objectmanager tests when building with MSVC
   (right now these tests don't work on Windows anyway, so the fact
    that MSVC can't even build them properly is irrelevant;
    most likely gdbus-codegen needs changes to put _GLIB_EXTERN
    before each function)