This fixes the issue "Custom onHealthCheck function is intermittently called" reported at apollographql#3999.
This fix ensures the healthcheck handling code does not carry on executing and waits for the returned promise to be resolved or rejected.

The tests validates that a healthcheck endpoint exists and correctly handles promises returned from the callback.

This commit also moves the 'mock server'-like code (responsible for transforming lambda response into HTTP response) has been moved to a common location where it is reused in the newly created apollo-server-lambda/src/ApolloServer.test.ts.