Disable checkpoint during tests.

[paddycarver]

We don't need to reach out to checkpoint when running tests, so disable
that information. Fixes #640.

Also remove our logging override, as we've updated to a version of
go-plugin that doesn't require that anymore. See also #661.

[kmoe]

Whether the SDK needs to reach out to Checkpoint during tests depends on the following environment variables:

• TF_ACC_TERRAFORM_VERSION
• TF_ACC_TERRAFORM_PATH

If neither of these is set, and if there is no version of terraform on the system $PATH, plugintest will fall back to installing the latest available version of Terraform via tfinstall.LatestVersion, with forceCheckpoint set to true (see https://github.com/hashicorp/terraform-plugin-sdk/blob/master/internal/plugintest/config.go#L42). This means the value of CHECKPOINT_DISABLE is ignored, so the latest version of Terraform can always be found.

[ghost]

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