twister: Add sysbuild boolean to platform definitions

More complex platforms require sysbuild to use always, even for
such "simple" samples like hello_world. Such platforms can have
`sysbuild: true` entry in their board_name.yaml used by twister.
Using such entry will tell twister, that sysbuild must always be used
on a given platform.
Twister is aligned to have information about need of sysbuild at
instance (platform + suite) level (was only at suite level before).
Instance.sysbuild is true whenever a test suite or a platform requires
sysbuild.
Twister pytest unit tests are aligned with changes.

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>