payment: add test for DirectPaymentLeavingStateCallback

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment: add tests for PluginOperation

This also fixes a bug in handling RuntimeException.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
payment: add tests for DirectPaymentOperation

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>