tests: drivers: gpio_basic_api: fix deprecated API level test

Some boards don't support level interrupts; respect their rejection of
the configuration.

Also correct the code intended to disable the interrupt from within
the callback when level triggers are tested.  Note that the legacy
call emulation does not work: it's necessary to add a flag that causes
the interrupt to be disabled.

Also improve a diagnostic and fix the exit path for a failure detected
before the callback was installed.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>