setexpr: Correct buffer overflow bug and enable tests

At present when more than one substitution is made this function
overwrites its buffers. Fix this bug and update the tests now that they
can pass.

Also update the debug code to show all substrings, since at present it
omits the final one.

Fixes: 855f18ea ("setexpr: add regex substring matching and substitution")
Signed-off-by: Simon Glass <sjg@chromium.org>