[testing] Support direnv to simplify usage of test tooling

[marun]

Why this should be merged

Adds a .envrc file for use with direnv enables automatic configuration of paths and environment variables to simplify usage of test tooling. For example, ginkgo and tmpnetctl can be invoked directly and without having to specify the --avalanchego-path or --plugin-dir flags. It also avoids the use of global paths for those of us that develop concurrently on multiple local worktrees.

How this works

• Add a .envrc file to support automatic shell configuration for those who have direnv installed
• Ensures backwards-compatible behavior for those who don't know about or don't want to use direnv

How this was tested

• CI to avoid regression
• Manually for direnv usage

Need to be documented in RELEASES.md?

N/A