Merge pull request #772 from SimoneDutto/fix-flaky-TestAcc_CustomResourcesAddedToPlanMicrok8s

#772

There is a Juju timing issue by which if the app is created and after it the resource is updated sometimes the update is not applied. This is most likely an issue that is not affecting any user because it is just a scenario presented in our tests.The fix is just to sleep between step 1 and step 2.

For future reference:
- this seems to be a very specific issue with the way we update resources, when the test is failing and you check the model you see something like this:
```
runner@pkrvmxyh4eaekms:~/work/terraform-provider-juju/terraform-provider-juju$ juju resources test-app
Resource Supplied by Revision
coredns-image store 65

[Updates Available]
Resource Revision
coredns-image 0
```
I think the reason is that the application is created and the resource update is queue but not applied because the application has not reached a specific state where you can actually update the resource.
- i couldn't reproduce the issue locally
- we can't wait for the resource to be populated in `Create` because we don't have the resource specified all of the time in our plan.
- we can't wait for the application to reach a non `waiting` status, because it takes a long time for most charms, and it will just create longer tests.

# QA
i couldn't reproduce the issue even with this very specific plan.

```terraform
terraform {
 required_providers {
 juju = {
 source = "juju/juju"
 }
 }
}

variable "custom_resource" {
 type = bool
 default = false
}

provider "juju" {}

resource "juju_model" "development" {
 name = "dev2"
}


resource "juju_application" "this" {
 name = "this"

 model = juju_model.development.name

 charm {
 name = "coredns"
 channel = "latest/stable"
 }

 resources = var.custom_resource == true ? {
 "coredns-image" = "docker.io/coredns/coredns:latest"
 } : null
}
```

`terraform apply -var custom_resource=false -auto-approve && terraform apply -var custom_resource=true -auto-approve`

Author: jujubot

internal/provider/resource_application_test.go

@@ -480,6 +480,18 @@ func TestAcc_CustomResourcesAddedToPlanMicrok8s(t *testing.T) {
 					resource.TestCheckNoResourceAttr("juju_application.this", "resources"),
 				),
 			},
+			// In the next step we verify the plan has no changes. First waiting 30 seconds
+			// to avoid a race condition in Juju where updating the resource revision too
+			// quickly means that the change doesn't take immediate effect.
+			{
+				Config: testAccResourceApplicationWithoutCustomResources(modelName, "latest/stable"),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckNoResourceAttr("juju_application.this", "resources"),
+				),
+				PreConfig: func() {
+					time.Sleep(30 * time.Second)
+				},
+			},
 			{
 				// Add a custom resource
 				Config: testAccResourceApplicationWithCustomResources(modelName, "latest/stable", "coredns-image", "ghcr.io/canonical/test:6a873fb35b0170dfe49ed27ba8ee6feb8e475131"),