kunit: use KUnit defconfig by default

To improve the usability of KUnit, defconfig is used
by default if no kunitconfig is present.

 * https://bugzilla.kernel.org/show_bug.cgi?id=205259



Fixed up minor merge conflicts - Shuah Khan <skhan@linuxfoundation.org>

Signed-off-by: Vitor Massaru Iha <vitor@massaru.org>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>