kunit: tool: handle when .kunit exists but .kunitconfig does not

Right now .kunitconfig and the build dir are automatically created if
the build dir does not exists; however, if the build dir is present and
.kunitconfig is not, kunit_tool will crash.

Fix this by checking for both the build dir as well as the .kunitconfig.

NOTE: This depends on commit 5578d008 ("kunit: tool: fix running
kunit_tool from outside kernel tree")

Link: https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git/commit/?id=5578d008d9e06bb531fb3e62dd17096d9fd9c853


Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>