Integration tests

[anthonyhastings]

• Adding start-apollo-server-test command to run a removable container for running tests.
• Adding server/tsconfig.build.json specifically for building assets (this config ignores test related files). The original tsconfig.json can be used for type-checking and apply to all files.
• Updating server/src/server.mts to expose the server as well as the start/listen method; this allows us to use the server within tests without actually binding it to a port.
• Adding jest and ts-jest packages to run typescript files in Jest.
• Adding npm scripts to run tests in ESM mode and build artifacts beforehand.
• Adding tests for Buyer and Order queries and mutations; they stub the data source class and return responses accordingly.

More Information:

• https://www.apollographql.com/docs/apollo-server/testing/testing/
• https://www.apollographql.com/docs/apollo-server/api/apollo-server/#executeoperation