Modify testread example to loop using same context.

Modify testread example to loop using same context.

There's been a problem seen where open/read/close a number of times causes
open failures eventually.  This program has been modified to create the
context once and then loop requesting file names to open/read/close.

This program also demonstrates the current error in cli_read() where it
returns an error instead of length 0 upon end of file.

Derrell
(This used to be commit 9d75ea577b407ccab59196760d376831062a3ab5)