Add grpc error intercept test

[ciarams87]

Proposed changes

We added error intercept location blocks for cases where the upstream is set as gRPC - see https://www.nginx.com/blog/deploying-nginx-plus-as-an-api-gateway-part-3-publishing-grpc-services/#Responding-to-Errors for details. This adds a test to make sure the headers are being applied.

It also updates the test tls cert and key as it was accidentally created with a one month validity.

Checklist

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• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork