Add AWS Nitro Enclaves test execution to the CI system

[dreemkiller]

Requested feature
Currently, the CI builds for SGX, builds for TrustZone, and runs the tests for TrustZone. We need to add build and run for AWS Nitro Enclaves.

Motivation
Unless the AWS nitro platform is being built and run in the CI system, the AWS Nitro support will atrophy and suffer from bitrot.

Additional context
We need to modify the CI system to start on an AWS-Nitro-supported system. To be clear, I'm not sure how to do this with the current CI system.

[dreemkiller]

It appears that adding AWS Nitro builds to veracruz can be done fairly easily: edit buildspec.yml to add

make nitro-veracruz-server-test-dry-run
make nitro-veracruz-test-dry-run

to the build commands.

It is likely that the docker image being used for the CI would need to be updated, though.

Actually running the Nitro tests is a different matter. This would require modifying the CodeBuild environment to be run on an EC2 instance that supports AWS Nitro Enclaves, which I'm not sure is possible.

[gbryant-arm]

Done in #258 and superseded by #514