On macOS, closefrom() only sets the close-on-exec flag, so we cannot
check whether all file descriptors were closed, which means that if
on entry our file descriptor table was filled after the 4th file
descriptor, then we might fail the assertions for the flags for odd
file descriptors which we expect to be closed.

This can easily happen when running the test suite in parallel mode
with «make -j8 check» for example.

Closes: #23