This commit updates GEMINI.md to include instructions for building and
running Nexus tests. Nexus is a test framework that enables faster and
more scalable network simulations within a single process.
The updated documentation provides:
- A brief overview of the Nexus test framework.
- The command to build Nexus tests using 'tests/nexus/build.sh'.
- Commands to run all Nexus tests or a specific test using
'tests/nexus/run_nexus_tests.sh'.
- A reference to 'tests/nexus/README.md' for further details.