REGTESTS: add test on backend switching rules selection

REGTESTS: add test on backend switching rules selection

Create a new test to ensure that switching rules selection is fine.
Currently, this checks that dynamic backend switching works as expected.
If a matching rule is resolved to an unexisting backend, the default
backend is used instead.

This regtest should be useful as switching-rules will be extended in a
future set of patches to add new abilities on backends, linked to
dynamic backend support.