This commit reworks the snippet test to more comprehensively test the
different processing modes of snippets.
The following testcases have been added:
1. Initial state with no snippet applied
2. Applying `foo` snippet from the default application snippet root
3. Applying `bar` snippet from an extra snippet root
4. Sequentially applying `foo` and `bar` snippets to test the snippet
processing order
5. Sequentially applying `bar` and `foo` snippets to test the snippet
processing order.
Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>
f1a18022be