Restrict running test using non-existing backend.

[zie1ony]

When running without any backends:

$ cargo odra test -b casper

All test passes, but it should fail with proper message.

[kubaplas]

@zie1ony
Couldn't reproduce. When running on non-existing backend:

$ cargo odra test -b casper_lolol                                                                                                                               
No such backend.

Without any backends:

$ cargo odra test -b casper_lolol                                                                                                                              ✘ 1
No backends configured