all: Add deferred action testing support (plan checks, version check, and CLI options)

[austinvalle]

This PR adds a couple different things all towards supporting testing deferred actions:

• Two plan checks that enable inspecting the new deferred changes field in the plan JSON, ref: tfjson: Add DeferredChanges and Complete to Plan JSON terraform-json#123
• A new version check that ensures a test is only run on pre-release builds of Terraform
• A new TestCase field for enabling a limited set of flags, in this scenario, the -allow-deferral flag, ref: tfexec: Add -allow-deferral experimental options to Plan and Apply commands terraform-exec#447
• I also updated the terraform-plugin-go version to v0.23.0 to help with testing the new plan checks, ref: tfprotov5+tfprotov6: Add deferred action support to related RPCs terraform-plugin-go#403
  • This required updating some of the testprovider to implement the now required methods from terraform-plugin-go

[SBGoods]

LGTM! Only one non-blocking question about a potential new check.

[SBGoods]

Would it be useful to have a ExpectNoDeferredChange plan check assertion for a single resource?

[austinvalle]

Good question! Off the top of my head I can't think of a specific testing flow that would need a negative check like that.

My initial thought is that most tests would follow the general flow of testing a single resource:

• Test step 1: Check a resource is causing a plan to be deferred
• Test step 2: Check there are no more deferred changes (i.e. no more rounds needed)

I'm not sure if there is a workflow where a test may want to end before all deferred changes are satisfied, but maybe more provider developer usage will let us know if that'd be beneficial.

[bendbennett]

LGTM 🚀