When writing tests to check the output from commands it is useful to be
able to check the output line by line using an assertion. Add helper
macros to support this and to check that there is no unexpected trailing
data.

Also some commands produce a dump using print_buffer(). Add a way to check
that the correct number of bytes are dumped (ignoring the actual
contents).

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