Expand EC2 CLI testcase coverage

[rehanshah17]

• describe-ipam-scopes.rst
• describe-scheduled-instance-availability.rst
• describe-snapshot-attribute.rst
• describe-spot-fleet-requests.rst
• describe-tags.rst
• describe-vpc-endpoint-connections.rst
• describe-vpc-endpoint-service-permissions.rst
• get-serial-console-access-status.rst
• modify-security-group-rules.rst
• modify-subnet-attribute.rst
• modify-vpc-endpoint-service-configuration.rst

[zyang37]

Thanks for the PR. All cases seem to have merge conflicts. And I have some questions:

1. Where do these test cases come from?
2. The outputs seem to be generated from an emulator?