This test assumed that it was invoked in an empty directory,
which is true when run from the Makefile, and so would fail
if run standalone. In order to allow the test to work when
run from any directory, create a sub directory "dir-walk"
and chdir() into this directory while running the tests.

Also, add some additional tests.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>