wip: TargetPool failing test for HTTPS HC

Test that we can reference our new `google_compute_https_health_check`
resource. This currently fails for two reasons.

1. It seems that we're not able to provide more than one health check in the
   same request:

    --- FAIL: TestAccComputeTargetPool_health_checks (4.37s)
            testing.go:135: Step 0 error: Error applying: 1 error(s) occurred:

                    * 1 error(s) occurred:

                    * 1 error(s) occurred:

                    * Error creating TargetPool: googleapi: Error 413: Value for field 'resource.healthChecks' is too large, fieldSizeTooLarge

2. If we comment out the HTTP health check then it produces an error because
   the HTTPS health check comes from a different API version.

    --- FAIL: TestAccComputeTargetPool_health_checks (4.63s)
            testing.go:135: Step 0 error: Error applying: 1 error(s) occurred:

                    * 1 error(s) occurred:

                    * 1 error(s) occurred:

                    * Error creating TargetPool: googleapi: Error 400: Invalid value for field 'healthChecks': 'https://www.googleapis.com/compute/beta/projects/skilful-card-92510/global/healthChecks/foobar'.  URL version does not match the API version., invalid

I wonder whether it would be easier on both accounts to add each health
check individually.

builtin/providers/google/resource_compute_target_pool_test.go

@@ -43,7 +43,10 @@ func TestAccComputeTargetPool_health_checks(t *testing.T) {
  testAccCheckComputeTargetPoolExists(
  "google_compute_target_pool.foobar", &targetPool),
  testAccCheckComputeTargetPoolHealthChecks(
- []string{"google_compute_http_health_check.foobar"}, &targetPool),
+ []string{
+ "google_compute_http_health_check.foobar",
+ "google_compute_https_health_check.foobar",
+ }, &targetPool),
  ),
  },
  },
@@ -137,8 +140,15 @@ resource "google_compute_http_health_check" "foobar" {
  name = "foobar"
  description = "Resource created for Terraform acceptance testing"
 }
+resource "google_compute_https_health_check" "foobar" {
+ name = "foobar"
+ description = "Resource created for Terraform acceptance testing"
+}
 resource "google_compute_target_pool" "foobar" {
  name = "terraform-test"
  description = "Resource created for Terraform acceptance testing"
- health_checks = [ "${google_compute_http_health_check.foobar.self_link}" ]
+ health_checks = [
+ "${google_compute_http_health_check.foobar.self_link}",
+ "${google_compute_https_health_check.foobar.self_link}",
+ ]
 }`