Ensure rest snippets task output directory is cleaned before execution (#78653)

build-tools-internal/src/main/groovy/org/elasticsearch/gradle/internal/doc/DocsTestPlugin.groovy

@@ -66,6 +66,9 @@ class DocsTestPlugin implements Plugin<Project> {
         TaskProvider<RestTestsFromSnippetsTask> buildRestTests = project.tasks.register('buildRestTests', RestTestsFromSnippetsTask) {
             defaultSubstitutions = commonDefaultSubstitutions
             testRoot.convention(restRootDir)
+            doFirst {
+                project.delete(restRootDir)
+            }
         }
 
         // TODO: This effectively makes testRoot not customizable, which we don't do anyway atm

build-tools-internal/src/main/groovy/org/elasticsearch/gradle/internal/doc/RestTestsFromSnippetsTask.groovy

@@ -68,7 +68,6 @@ class RestTestsFromSnippetsTask extends SnippetsTask {
     RestTestsFromSnippetsTask(ObjectFactory objectFactory) {
         testRoot = objectFactory.directoryProperty()
         TestBuilder builder = new TestBuilder()
-        doFirst { outputRoot().delete() }
         perSnippet builder.&handleSnippet
         doLast builder.&checkUnconverted
         doLast builder.&finishLastTest