tests: settings: Performance test for settings

This adds a on target performance test for Settings SS. Using this test
performance of the Setting SS + NVS/ZMS backend can be benchmarked.

The test repeatedly write 128 settings entries. Each setting entry
has a size of 4 bytes, and path length of 16 bytes (excluding the
null-terminator).

The test has two variants, with and without Bluetooth scan running.
This is useful to benchmark performance along with some component
of BT subsystem activated.

The test could be enhanced in future to include or create combinations
of different functionalities running, when agreesive store operations
are happening.

Signed-off-by: Omkar Kulkarni <omkar.kulkarni@nordicsemi.no>