Enable GIO tests on Windows
- Remove the non-Windows-only condition for subdir('tests').
- Add libiphlpapi, libws2_32 and libsecur32 deps, needed for W32 tests.
- 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).