Make gio/file tests work on more than just Linux
Following on from !1981 (comment 1054748), the tests added in that MR were added for Linux only, due to the need for them to pass CI quickly to fix a security issue. It would be good to open them up so they run on other platforms too as much as possible.
Mostly, the tests should work OK on other POSIX platforms. Apart from the symlink tests, they should work on Windows. Special handling will be needed for symlinks themselves on Windows.