Moves plan generator from utils package to testing.

Cherry picked commit, fixed typo in directory name and removed config
change from test as it won't work here. Will be added later.

internal/provider/resource_application_test.go

@@ -9,6 +9,8 @@ import (
 
 	"github.com/hashicorp/terraform-plugin-testing/helper/acctest"
 	"github.com/hashicorp/terraform-plugin-testing/helper/resource"
+
+	internaltesting "github.com/juju/terraform-provider-juju/internal/testing"
 )
 
 func TestAcc_ResourceApplication(t *testing.T) {
@@ -137,6 +139,37 @@ func TestAcc_ResourceApplication_Updates(t *testing.T) {
 	})
 }
 
+// TestAcc_ResourceApplication_UpdatesRevisionConfig will test the revision update that have new config parameters on
+// the charm. The test will check that the config is updated and the revision is updated as well.
+func TestAcc_ResourceApplication_UpdatesRevisionConfig(t *testing.T) {
+	if testingCloud != LXDCloudTesting {
+		t.Skip(t.Name() + " only runs with LXD")
+	}
+	modelName := acctest.RandomWithPrefix("tf-test-application")
+	appName := "github-runner"
+	resource.Test(t, resource.TestCase{
+		PreCheck:                 func() { testAccPreCheck(t) },
+		ProtoV6ProviderFactories: frameworkProviderFactories,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccResourceApplicationWithRevisionAndConfig(modelName, appName, 88, ""),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("juju_application."+appName, "model", modelName),
+					resource.TestCheckResourceAttr("juju_application."+appName, "charm.#", "1"),
+					resource.TestCheckResourceAttr("juju_application."+appName, "charm.0.name", appName),
+					resource.TestCheckResourceAttr("juju_application."+appName, "charm.0.revision", "88"),
+				),
+			},
+			{
+				Config: testAccResourceApplicationWithRevisionAndConfig(modelName, appName, 95, ""),
+				Check: resource.ComposeTestCheckFunc(
+					resource.TestCheckResourceAttr("juju_application."+appName, "charm.0.revision", "95"),
+				),
+			},
+		},
+	})
+}
+
 func TestAcc_CharmUpdates(t *testing.T) {
 	modelName := acctest.RandomWithPrefix("tf-test-charmupdates")
 
@@ -290,6 +323,40 @@ func testAccResourceApplicationBasic(modelName, appName string) string {
 	}
 }
 
+func testAccResourceApplicationWithRevisionAndConfig(modelName, appName string, revision int, configParamName string) string {
+	return internaltesting.GetStringFromTemplateWithData(
+		"testAccResourceApplicationWithRevisionAndConfig",
+		`
+resource "juju_model" "{{.ModelName}}" {
+  name = "{{.ModelName}}"
+}
+
+resource "juju_application" "{{.AppName}}" {
+  name  = "{{.AppName}}"
+  model = juju_model.{{.ModelName}}.name
+
+  charm {
+    name     = "{{.AppName}}"
+    revision = {{.Revision}}
+    channel  = "latest/edge"
+  }
+
+  {{ if ne .ConfigParamName "" }}
+  config = {
+    {{.ConfigParamName}} = "{{.ConfigParamName}}-value"
+  }
+  {{ end }}
+
+  units = 1
+}
+`, internaltesting.TemplateData{
+			"ModelName":       modelName,
+			"AppName":         appName,
+			"Revision":        fmt.Sprintf("%d", revision),
+			"ConfigParamName": configParamName,
+		})
+}
+
 func testAccResourceApplicationUpdates(modelName string, units int, expose bool, hostname string) string {
 	exposeStr := "expose{}"
 	if !expose {

internal/testing/plangenerator.go

@@ -0,0 +1,84 @@
+// Copyright 2024 Canonical Ltd.
+// Licensed under the Apache License, Version 2.0, see LICENCE file for details.
+
+package testsing
+
+import (
+	"bytes"
+	"text/template"
+)
+
+type TemplateData map[string]string
+
+// GetStringFromTemplateWithData returns a string from a template with data.
+//
+// Here is the example usage:
+// templateStr := GetStringFromTemplateWithData(
+//
+//	"testAccResourceApplicationWithRevisionAndConfig",
+//	`
+//
+//	resource "juju_model" "this" {
+//	 name = "{{.ModelName}}"
+//	}
+//
+//	resource "juju_application" "{{.AppName}}" {
+//	 name  = "{{.AppName}}"
+//	 model = juju_model.this.name
+//
+//	 charm {
+//	   name     = "{{.AppName}}"
+//	   revision = {{.Revision}}
+//	   channel  = "latest/edge"
+//	 }
+//
+//	 {{ if ne .ConfigParamName "" }}
+//	 config = {
+//	   {{.ConfigParamName}} = "{{.ConfigParamName}}-value"
+//	 }
+//	 {{ end }}
+//
+//	 units = 1
+//	}
+//
+//	`, utils.TemplateData{
+//					"ModelName":       "test-model",
+//					"AppName":         "test-app"
+//					"Revision":        fmt.Sprintf("%d", 7),
+//					"ConfigParamName": "test-config-param-name",
+//				})
+//
+// The templateStr will be:
+//
+//	resource "juju_model" "this" {
+//	  name = "test-model"
+//	}
+//
+//	resource "juju_application" "test-app" {
+//	  name  = "test-app"
+//	  model = juju_model.this.name
+//
+//	  charm {
+//	    name     = "test-app"
+//	    revision = 7
+//	    channel  = "latest/edge"
+//	  }
+//
+//	  config = {
+//	    test-config-param-name = "test-config-param-name-value"
+//	  }
+//
+//	  units = 1
+//	}
+func GetStringFromTemplateWithData(templateName string, templateStr string, templateData TemplateData) string {
+	tmpl, err := template.New(templateName).Parse(templateStr)
+	if err != nil {
+		panic(err)
+	}
+	var tpl bytes.Buffer
+	err = tmpl.Execute(&tpl, templateData)
+	if err != nil {
+		panic(err)
+	}
+	return tpl.String()
+}