TESTING rpc api group allowed

[macfarla]

PR description

This PR just adds the TESTING group to the acceptable list.
Refer to #9838 which has an early implementation of a testing endpoint

Fixed Issue(s)

latest hive run failed with

Failed to start Besu: Invalid value for option '--rpc-http-api': invalid entries found [TESTING]
Invalid value for option '--rpc-http-api': invalid entries found [TESTING]

Thanks for sending a pull request! Have you done the following?

• Checked out our contribution guidelines?
• Considered documentation and added the doc-change-required label to this PR if updates are required.
• Considered the changelog and included an update if required.
• For database changes (e.g. KeyValueSegmentIdentifier) considered compatibility and performed forwards and backwards compatibility tests

Locally, you can run these tests to catch failures early:

• spotless: ./gradlew spotlessApply
• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest
• reference tests: ./gradlew ethereum:referenceTests:referenceTests
• hive tests: Engine or other RPCs modified?

[Copilot]

Pull request overview

Adds the TESTING JSON-RPC API group to the set of accepted --rpc-http-api values to prevent startup failures when TESTING is configured.

Changes:

• Added TESTING to the RpcApis enum so it is treated as a valid RPC API group.

[matkt]

LGTM