binman: Adjust size of test SPL binary

binman: Adjust size of test SPL binary

This is only 3 bytes long which is not enough to hold two symbol values,
needed to test the binman symbols feature. Increase it to 15 bytes.

Using very small regions is useful since we can easily compare them in
tests and errors are fairly easy to diagnose.

Signed-off-by: Simon Glass <sjg@chromium.org>