[Task]: Integrate the runtime-tools validation suite into the repo

[michael-duren]

What needs to be done

Bring the opencontainers/runtime-tools validation suite into the project using
the integration approach chosen during research (vendor / git submodule /
fetch-in-CI). Get the suite building and able to invoke box as RUNTIME.

Includes:

• Adding the suite to the repo via the chosen mechanism (and documenting how to
  update/pin it).
• Ensuring the runtimetest helper and validation test binaries build in our
  environment (Go toolchain, bats dependency).
• A reproducible way to point the suite at the locally-built box binary.

Acceptance criteria

• runtime-tools validation suite is present/pinned in the repo via the chosen approach
• The suite and runtimetest helper build cleanly locally
• The suite can be pointed at the box binary and at least starts executing tests
• How to update the pinned suite version is documented
• .gitignore / build artifacts handled so the suite doesn't pollute the tree

Parent / tracking issue

Part of the OCI runtime validation epic.

Blocked by

• Research the OCI runtime-tools validation suite & CLI contract

Rough size

M — a day or two