tests: drivers: watchdog: wdt_error_cases: Add test scenario

Add test scenario that verifies correct error code
returned by wdt_setup() when called just after wdt_disable().

Scenario:
- configure WDT timeout;
- start WDT by calling wdt_setup();
- remove all WDT timeouts by calling wdt_disable();
- confirm wdt_setup() returns error code due to lack
  of configured WDT timeouts.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>