Replace `@hapi/shot` with `light-my-request`

Rework @loopback/testlab to use light-my-request for stubbing HTTP requests & responses.

Arguments for the change:

• light-my-request has larger community of contributors (45 vs 24)
• light-my-request has more activity (compare https://github.com/fastify/light-my-request/graphs/contributors?from=2020-01-01&to=2020-09-18&type=c to https://github.com/hapijs/shot/graphs/contributors?from=2020-01-01&to=2020-09-18&type=c)
• At some point in the past, @hapi/shot did not support all LTS versions of Node.js, we had to stay at an old major version. Nowadays they don't limit the supported versions via engines.node, but their CI is configured to test only on Node.js 12+.