chdir(): Fix error path, and add test

This patch fixes chdir() on a normal file, which used to succeed (!?),
but now will fail as it should, with ENOTDIR.

The patch also adds an exhaustive test for chdir's success and error cases.
Before the latest chdir() patches, most of these tests would fail, and now
all of them succeed.

This test is standard C++ & Posix code, so it can be run also on Linux.
This is important for verifing that whatever we expect from OSv, Linux
really does the same.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>