feat: add ecomm-test-courses.json

[pshiu]

New test file for testing common ecommerce usecases.

Usage: run make dev.up.lms+studio+ecommerce then create-courses --studio --ecommerce ecomm-test-courses.json in your shell from the course-generator directory in the root of the repository.

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I've completed each of the following or determined they are not applicable:

• [X] Made a plan to communicate any major developer interface changes (or N/A)