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)
giomodule test needed symbol visibility pragmas added. This is needed on
Windows anyway, so it's better to do it this way rather than disabling
-fvisibility=hidden for the test modules.
