This function can be used to check if a Provisioner has been marked for testing within the ACC_TEST_PROVISIONERS environment variable.
While testing I found that the shell acceptance test were also running when trying to run powershell tests.
Before change
```
⇶ ACC_TEST_BUILDERS=amazon-ebs ACC_TEST_PROVISIONERS=powershell go test -v ./provisioner/shell/... -timeout=1h
=== RUN
TestShellProvisioner/testing_amazon-ebs_builder_against_shell_provisioner
2020/04/06 15:18:12 ui: amazon-ebs: output will be in this color.
2020/04/06 15:18:12 ui:
2020/04/06 15:18:12 Build debug mode: false
2020/04/06 15:18:12 Force build: false
2020/04/06 15:18:12 On error:
2020/04/06 15:18:12 Preparing build: amazon-ebs
2020/04/06 15:18:12 Waiting on builds to complete...
2020/04/06 15:18:12 Starting build run: amazon-ebs
2020/04/06 15:18:12 Running builder: amazon-ebs
```
After changes
```
⇶ ACC_TEST_BUILDERS=amazon-ebs ACC_TEST_PROVISIONERS=powershell go test -v ./provisioner/shell/... -timeout=1h
--- SKIP: TestShellProvisioner (0.00s)
provisioners.go:88: Provisioner "shell" not defined in ACC_TEST_PROVISIONERS
```
