Automation: Modify parallel test execution value from 6 to 12

Background / User story

We have bumped up the LambdaTest number of parallel tests from 6 to 12.

What to change

• Design: N/A
• Research: N/A
• Spec: N/A
• Legal: N/A
• Development:
  • Modify parallel test execution value from 6 to 12 in the conf file.

Hints for testers

End to end tests should be passing locally: https://gitlab.com/eyeo/adblockplus/abpui/adblockplusui#end-to-end-testing

Hints for translators

N/A