Enable NAPI tests to run with locally run gltestserver

[ShahanaFarooqui]

This PR updates the test infrastructure for the Node.js NAPI bindings to properly support running tests against a locally run gltestserver. Previously, tests attempted to connect to remote Greenlight services. Now, tests can be run against a local regtest environment for faster, more reliable, and isolated testing.

Key Changes

• Environment Configuration
• Added test helper functions
• Adjusted current tests to run with local gltestserver
• Documentation Updates

Limitations

• send / receive tests skipped: These operations require an LSPS2 peer connection. The necessary connectPeer command is not yet implemented, preventing these tests from running.
• Skipping onchainSend due to timeout: The onchainSend method currently hangs indefinitely, causing the related tests to time out.

This PR establishes the foundational test setup required for Issue #677 by enabling Jest integration tests to run against a local gltestserver. This immediate solution allows for reliable testing while laying the groundwork for the next phase: fully integrating with the existing Python testing infrastructure (pyln.testing + gl-testing).

[ShahanaFarooqui]

Closing in favour of #681, which replaces the custom helper functions in this PR with the existing gl-testing fixtures directly.