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