tfversion: Add SkipIfNotAlpha version check

[austinvalle]

Since we've recently added beta prereleases to our acceptance testing, we started getting test failures for experimental features like deferred actions that use the SkipIfNotPrerelease version check. That's because the only prerelease builds that get experimental features (like deferred actions) are alpha and source builds.

None of our CI uses source builds, so I just created a new SkipIfNotAlpha version check we can use for experimental features, while allowing beta builds to run in our CI successfully 👍🏻

[austinvalle]

You can see some example test failures in our recent dependabot PRs when it fails to skip properly for 1.10.0-beta1: https://github.com/hashicorp/terraform-plugin-testing/actions/runs/11742344241/job/32712881987?pr=387

--- FAIL: Test_ExpectDeferredChange_Reason_Match (0.51s)
    expect_deferred_change_test.go:24: Step 1/1 error: Error running pre-apply plan: -allow-deferral is only
available in experimental Terraform builds: experiments are not enabled in version 1.10.0-beta1, as it's not
an alpha or dev build

[austinvalle]

After updating our CI to Terraform 1.10.0-rc1, I found an invalid test setup due to a new error from the introduction of ephemeral variables in the apply command, which rejects variables not defined in configuration during apply.

Example (names are different because I was testing it locally)

│ Error: Can't set variable when applying a saved plan
│ 
│   on test.auto.tfvars.json line 3:
│    3:     "var_two": false
│ 
│ The variable var_two cannot be set using the -var and -var-file options when applying a saved plan file, because it is neither ephemeral nor has it been declared during the plan operation.
│ To declare an ephemeral variable which is not saved in the plan file, use ephemeral = true.

I don't think this is a bug in Terraform core necessarily, since it already warns users about this misconfiguration, for example, during plan:

│ Error: Can't set variable when applying a saved plan
│ 
│   on test.auto.tfvars.json line 3:
│    3:     "var_two": false
│ 
│ The variable var_two cannot be set using the -var and -var-file options when applying a saved plan file, because it is neither ephemeral nor has it been declared during the plan operation.
│ To declare an ephemeral variable which is not saved in the plan file, use ephemeral = true.

I think in our case here, the test was simply not setup properly, and the new version of Terraform is confirming that. So the fix here is just correctly declaring the variables in the configuration 👍🏻