Unit tests are mainly for some complicated logics in Terraform code. We can extract the expressions containing complicated logics to local blocks, and perform tests against local expressions. Unit tests are optional, if they are composed please put the test code in test/unit directory. All go test code files in test/unit directory must be put in unit package:
package unit
import ...Specifically, create a symlink in unit-test-fixture directory pointing to variable.tf and locals.tf, then unit-test-fixture will be a Terraform module which contains only variable and locals block. Unit tests can be created for this module.
Make sure that unit tests does not involve any resource or data related to outside services. To verify the test results, an independent outputs.tf file can be added to the unit test folder to hold the outputs of the expressions in local need to be verified. resource and data that does not depend on outside services like null_resource, random_id can also be used.