test_runner: add getTestContext() public API

[MoLow]

Exposes getTestContext() function to access test context information from within tests and async operations.

My use case was exposing a pino logger that writes to test diagnostics, but there can be many other usecases:

import { prettyFactory } from 'pino-pretty';

const pretty = prettyFactory({ colorize: true  });
const asyncId = executionAsyncId();
const t = testResources.get(asyncId);
t.diagnostic(pretty(obj).replace(/\n$/, ''));