helper/resource: Check for existing provider configuration block in TestStep merged configuration

[bflad]

Closes #1064

Previously:

    --- FAIL: TestStepMergedConfig/teststep-externalproviders-config-with-provider-block-unquoted (0.00s)
        /Users/bflad/src/github.com/hashicorp/terraform-plugin-sdk/helper/resource/teststep_providers_test.go:643: unexpected difference:   (
              	"""
              	... // 6 identical lines
              	  }
              	}
            - 
            - 	provider "test" {}
            - 
              
              	provider test {}
              	... // 2 identical lines
              	"""
              )
    --- FAIL: TestStepMergedConfig/teststep-externalproviders-config-with-provider-block-quoted (0.00s)
        /Users/bflad/src/github.com/hashicorp/terraform-plugin-sdk/helper/resource/teststep_providers_test.go:643: unexpected difference:   (
              	"""
              	... // 6 identical lines
              	  }
              	}
            - 
            - 	provider "test" {}
            - 
              
              	provider "test" {}
              	... // 2 identical lines
              	"""
              )

[bendbennett]

LGTM 👍

[github-actions]

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