Stop creating new SM instance in TestProjectsExistingResourcesTest and test existing EN instance

[ocofaigh]

• Remove SM from tests/resources
• Update the TestProjectsExistingResourcesTest to use existing SM instance from common-permanent-resources.yaml (suggest to go with privateOnlySecMgrCRN)
• Test existing Event Notifications instance in this test too (will need fix that was added in fix: remove enforced validation for stacks use cases terraform-ibm-event-notifications#567)