KUnit: Docs: make start.rst example Kconfig follow style.rst

The primary change is that we want to encourage people to respect
KUNIT_ALL_TESTS to make it easy to run all the relevant tests for a
given config.

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