test/py: match prompt only at line boundaries

This prevents capture of command output from terminating early on boards
that use a simple prompt (e.g. "=> ") that appears in the middle of
command output (e.g. crc32's "... ==> 2fa737e0").

Reported-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>