Add ability to create Environment Variables for module tests

[sdrzazga-nsd]

Hi.

I'd like to request provider allows the definition of environment variables for module testing.

Use-cases

In Terraform Cloud it is possible to define for PMR modules with testing enabled to define environment variables.

Attempted Solutions

n/a

Proposal

Please consider to extends registry_module.test_config
or
introduce new resource like tfe_registry_module_tests_variable .