We never want tests to hit the real npm registry because this causes
test flakes. In addition, we set a sentinal "unset" value for
`NPM_CONFIG_REGISTRY` to ensure that all tests requiring npm go through
the test server.
This commit provides basic polyfill for "node:test" module. Currently
only top-level "test" function is polyfilled, all remaining functions from
that module throw not implemented errors.
