tests: add twister tests for CMake board and SoC extension

This commit adds new tests for verifying the functionality of the board
and SoC extension feature.

It does so by defining:
- A new CPU cluster on an existing SoC
- Two new board variants on top of an existing board

The new board variants are defined on top of the existing `native_sim`
board, so that the following native_sim board targets are available for
the test.
Existing board targets:
- native_sim/native
- native_sim/native/64

Extended board targets:
- native_sim/native/one
- native_sim/native/64/two

The new CPU cluster is defined for the existing `an521` SoC.
Existing CPU Clusters on an521:
- cpu0
- cpu1

New CPU Cluster:
- cputest

For SoC tests the mps2 board is used.
This means that for testing, the following board targets using the an521
SoC are:
- mps2/an521/cpu0
- mps2/an521/cpu1
- mps2/an521/cputest

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>