Update API tests to use a user account with limited permissions

Overview:
We've run into problems several times now where orders placed via edge-orders/mod-gobi don't work as expected due to permission issues.

In order to catch these problems earlier, we should update the API tests to utilize a user account with limited permissions as opposed to an admin user with "all" permissions.

We've agreed as a team to do this, and have already done so in several other places. This story is specific to mod-orders API tests - If there are other places requiring these changes they will be handled in separate stories.

Acceptance Criteria:

• The mod-orders API tests are updated such that all tests requests are made as a user with limited permissions (only those required by mod-orders)