patman: README documentation nits (unit test)

adjust instructions for the invocation of Patman's self test: the -t
flag appears to have a different meaning now, refer to the --test option
for the builtin unit test; adjust a directory location and make sure to
run the file which resides in the source directory

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>