test-fd-util: extend close_all_fds() test to trigger all fallback codepaths

test-fd-util: extend close_all_fds() test to trigger all fallback codepaths

This extends the close_all_fds() logic to overmount /proc with an empty
tmpfs, and/or to block close_range() via seccomp, so that we run the
test case for the function with the fallback paths.

This should make sure that we don't regress in limited environments or
older kernels.