cases/bbtests.py: test_bitbake_g(): Check base-files rather than busybox

cases/bbtests.py: test_bitbake_g(): Check base-files rather than busybox

It checked whether busybox is in task-depends.dot after run "bitbake -g
core-image-minimal", but busybox is not a must for core-image-minimal since it
is configurable:

VIRTUAL-RUNTIME_base-utils = ""
VIRTUAL-RUNTIME_base-utils-syslog = ""
VIRTUAL-RUNTIME_base-utils-hwclock  =  ""
VIRTUAL-RUNTIME_login_manager = "shadow"
VIRTUAL-RUNTIME_syslog = ""

So the case may fail when busybox is not present, check
base-files which is more reliable.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>