Fix path used when persisting working directory

[bendbennett]

Closes: #55

[bflad]

This now works as expected with configuring TestCase.WorkingDir and using TF_ACC_PERSIST_WORKING_DIR=1 🚀

I'm going to create a followup issue to consider some directory and file naming changes for usability:

$ tree /Users/bflad/test/testing55
/Users/bflad/test/testing55
├── step_1
│   ├── work2542351694
│   │   ├── terraform.tfstate
│   │   ├── terraform.tfstate.backup
│   │   ├── terraform_plugin_test.tf
│   │   └── tfplan
│   └── work3964272250
│       ├── terraform.tfstate
│       ├── terraform_plugin_test.tf
│       └── tfplan
├── step_2
│   └── work2542351694
│       ├── terraform.tfstate
│       ├── terraform.tfstate.backup
│       ├── terraform_plugin_test.tf
│       └── tfplan
├── step_3
│   └── work2542351694
│       ├── terraform.tfstate
│       ├── terraform.tfstate.backup
│       ├── terraform_plugin_test.tf
│       └── tfplan
├── work2542351694
│   ├── terraform.tfstate
│   ├── terraform.tfstate.backup
│   ├── terraform_plugin_test.tf
│   └── tfplan
├── work3964272250
│   ├── terraform.tfstate
│   ├── terraform.tfstate.backup
│   ├── terraform_plugin_test.tf
│   └── tfplan
└── work862501402
    ├── terraform.tfstate
    └── terraform_plugin_test.tf

11 directories, 25 files

As well as save multiple plan files to capture each intermediate plan, since all the tfplan are the post-apply-with-refresh no-op plans.

[github-actions]

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