Description of problem:
I have inspected the internal classes responsible for actual execution of Test Scenarios and noticed that whenever it is necessary to create a KieSession only Stateful will be created. So supplying a Stateless KieSession is simply not allowed - it will fail with this error message:

Test scenario runner could not find a stateful knowledge session with the name 'defaultSession'.

Version-Release number of selected component (if applicable):

BRMS 6.2.0
BPM Suite 6.2.0

How reproducible:
always

Steps to Reproduce:
1. Define Stateless session in business-central or directly in kmodule.xml
2. Configure Test Scenario with it
3. Run it

Actual results:
Test Scenarios can be configured only with Stateful session.

Expected results:
Test Scenarios can be configured with both, Stateful and Stateless sessions

Additional info:
Only Stateful is created
https://github.com/droolsjbpm/kie-wb-common/blob/6.3.x/kie-wb-common-services/kie-wb-common-services-backend/src/main/java/org/kie/workbench/common/services/backend/session/SessionServiceImpl.java#L54