test_runner: fix lazy test.assert accessor

[Renegade334]

The intention was for this to be a define-on-access lazy property, but the object passed to Object.defineProperty isn't a descriptor, so this never happens. As a result, the assert object is currently reconstructed on every access; test.assert === test.assert is false at present.

Changed to a conventional lazy getter, in alignment with the other node:test property accessors.