test: add E2E vaultcompat test for JWT auth flow

[lgfa29]

Test the JWT auth flow using real Nomad and Vault agents.

[tgross]

This looks great @lgfa29! It looks like there's a minor issue to mop-up with the changes in testutil, but LGTM once that's fixed.

[lgfa29]

Ops, syncing Nomad an Vault log level sounded like a good idea when running it locally but I didn't realize that off was used in CI 😅

I capped the log level for Vault to error but also added another env var to allow setting a log level specific for Vault if necessary.

[github-actions]

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