twister: doc: clarify harness keyword in test metadata

    in tests requiring sysbuild support being skipped.

harness: <string>

    A harness string needed to run the tests successfully. This can be as

    simple as a loopback wiring or a complete hardware test setup for

    sensor and IO testing.

    Usually pertains to external dependency domains but can be anything such as

    console, sensor, net, keyboard, Bluetooth or pytest.

    A harness keyword in the ``testcase.yaml`` file identifies a Twister

    harness needed to run a test successfully. A harness is a feature of

    Twister and implemented by Twister, some harnesses are defined as

    placeholders and have no implementation yet.

    A harness can be seen as the handler that needs to be implemented in

    Twister to be able to evaluate if a test passes criteria. For example, a

    keyboard harness is set on tests that require keyboard interaction to reach

    verdict on whether a test has passed or failed, however, Twister lack this

    harness implementation at the momemnt.

    The console harness tells Twister to parse a test's text output for a regex

    defined in the test's YAML file.

    Supported harnesses:

    - ztest

    - test

    - console

    - pytest

    Some widely used harnesses that are not supported yet:

    - keyboard

    - net

    - bluetooth

platform_key: <list of platform attributes>

    Often a test needs to only be built and run once to qualify as passing.

        automation setup based on "fixture" keyword. Some sample fixture names

        are i2c_hts221, i2c_bme280, i2c_FRAM, ble_fw and gpio_loop.

        Only one fixture can be defined per testcase.

        Only one fixture can be defined per testcase and the fixture name has to

        be unique across all tests in the test suite.

    pytest_root: <pytest directory> (default pytest)

        Specify a pytest directory which need to execute when test case begin to running,