ci: Split `ptest` for nightly and merge requests

Split the ptest for nightly/weekly and Merge request runs:
The intention is to only run short-duration tests for each
merge request raised and longer tests for nightly/weekly
* `Merge request` run includes:
  user-accounts-integration-tests, container-engine-integration-tests
  and k3s-integration-tests
* nightly/weekly run include parsec-simple-e2e-tests and all tests
  from `Merge request` run

Signed-off-by: Vikas Katariya <vikas.katariya@arm.com>