tests/resource/aws_appsync_datasource: Fix Lambda runtime
tests/resource/aws_appsync_datasource: Fix Lambda runtime

Previous output from acceptance testing:

```
--- FAIL: TestAccAwsAppsyncDatasource_Type_Lambda (25.53s)
    testing.go:568: Step 0 error: errors during apply:

        Error: Error creating Lambda function: InvalidParameterValueException: The runtime parameter of nodejs6.10 is no longer supported for creating or updating AWS Lambda functions. We recommend you use the new runtime (nodejs8.10) while creating or updating functions.
        	status code: 400, request id: 2ad8fdcd-78dd-11e9-82e1-0b28c3b9519b

          on /var/folders/v0/_d108fkx1pbbg4_sh864_7740000gn/T/tf-test220210354/main.tf line 21:
          (source code not available)
```

Output from acceptance testing:

```
--- PASS: TestAccAwsAppsyncDatasource_Type_Lambda (26.25s)
```

Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,terraform-providers/terraform-provider-aws,kjmkznr/terraform-provider-aws