helper/resource: Add terraform plan output to TRACE logging

[bflad]

Closes #698

For example:

$ TF_ACC=1 TF_LOG=TRACE go test -count=1 -run='TestTest_TestStep_ExternalProviders_DifferentVersions' -v ./helper/resource
...
2022-09-13T22:27:17.924-0400 [TRACE] sdk.helper_resource: Created plan with changes:
  test_terraform_plan=
  |
  | Terraform used the selected providers to generate the following execution
  | plan. Resource actions are indicated with the following symbols:
  |   + create
  |
  | Terraform will perform the following actions:
  |
  |   # null_resource.test will be created
  |   + resource "null_resource" "test" {
  |       + id = (known after apply)
  |     }
  |
  | Plan: 1 to add, 0 to change, 0 to destroy.
   test_name=TestTest_TestStep_ExternalProviders_DifferentVersions test_step_number=1 test_terraform_path=/opt/homebrew/bin/terraform test_working_directory=/var/folders/f3/2mhr8hkx72z9dllv0ry81zm40000gq/T/plugintest2492851432
...
2022-09-13T22:27:18.816-0400 [TRACE] sdk.helper_resource: Created plan with no changes: test_working_directory=/var/folders/f3/2mhr8hkx72z9dllv0ry81zm40000gq/T/plugintest2492851432 test_name=TestTest_TestStep_ExternalProviders_DifferentVersions test_step_number=2 test_terraform_path=/opt/homebrew/bin/terraform
...

[bendbennett]

LGTM

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