Extend VCS tests

[truszkowski]

Description of the change

Let's test more VCS integrations! Setting up ID, changing and mixing different (source code) providers.

If we had those tests, adding azure_devops.0.id (+ other VCSes) without DiffSuppressFunc/Computed would be ended with:

=== NAME  TestVCSIntegration/spacelift_stack/setting_up_ID/azure-devops-with-non-specified-integration-gdu23
    test_provider.go:30: Step 1/2 error: After applying this test step, the plan was not empty.
        stdout:

        Terraform used the selected providers to generate the following execution
        plan. Resource actions are indicated with the following symbols:
          ~ update in-place

        Terraform will perform the following actions:

          # spacelift_stack.test will be updated in-place
          ~ resource "spacelift_stack" "test" {
                id                               = "azure-devops-with-non-specified-integration-gdu23"
                name                             = "azure-devops-with-non-specified-integration-gdu23"
                # (15 unchanged attributes hidden)

              ~ azure_devops {
                  - id         = "azure-devops-default" -> null
                    # (2 unchanged attributes hidden)
                }
            }

        Plan: 0 to add, 1 to change, 0 to destroy.

Also, if we replace DiffSuppressFunc with Computed, it works!

Type of change

• [ ] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation (non-breaking change that adds documentation)

Related issues

Fix [#1]()

Checklists

Development

• [ ] Lint rules pass locally
• [ ] The code changed/added as part of this pull request has been covered with tests
• [ ] All tests related to the changed code pass in development
• [ ] Examples for new resources and data sources have been added
• [ ] Default values have been documented in the description (e.g., "Dummy: (Boolean) Blah blah. Defaults to false.)
• [ ] If the action fails that checks the documentation: Run go generate to make sure the docs are up to date

Code review

• [ ] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [ ] Pull Request is no longer marked as "draft"
• [ ] Reviewers have been assigned
• [ ] Changes have been reviewed by at least one other engineer