[TEST] Make MockMustacheScriptEngine less strict

Since elastic/elasticsearch#19621 MockScriptEngine is stricter and expects scripts to be defined before being used in tests. Because watcher makes heavy use of scripts without really need of custom logic, this commit changed the MockMustacheScriptEngine implementation so that it always returns the script's source as a result.

Original commit: elastic/x-pack-elasticsearch@09cffa9517

elasticsearch/x-pack/watcher/src/test/java/org/elasticsearch/script/MockMustacheScriptEngine.java

@@ -48,8 +48,8 @@ public class MockMustacheScriptEngine extends MockScriptEngine {
         if (script.contains("{{") && script.contains("}}")) {
             throw new IllegalArgumentException("Fix your test to not rely on mustache");
         }
-
-        return super.compile(name, script, params);
+        // We always return the script's source as it is
+        return new MockCompiledScript(name, params, script, null);
     }
 
     @Override