Extract similar usage of Resource.file.

Author: yuri-sergiichuk

testutil-gcloud/src/test/java/io/spine/testing/server/storage/datastore/TestDatastoresTest.java

@@ -105,9 +105,7 @@ void byResourceAtPath() {
         @Test
         @DisplayName("the service account resource")
         void byResource() {
-            Resource serviceAccount = Resource.file(
-                    SPINE_DEV_JSON, TestDatastoresTest.class.getClassLoader()
-            );
+            Resource serviceAccount = localResource(SPINE_DEV_JSON);
             Datastore datastore = TestDatastores.remote(serviceAccount);
             String projectId = datastore.getOptions()
                                         .getProjectId();
@@ -118,9 +116,11 @@ void byResource() {
     @Test
     @DisplayName("throw an `ISE` when can't properly parse account credentials from resource")
     void throwOnInvalidResource() {
-        Resource resource = Resource.file(
-                "random.json", TestDatastoresTest.class.getClassLoader()
-        );
+        Resource resource = localResource("random.json");
         assertThrows(IllegalStateException.class, () -> TestDatastores.remote(resource));
     }
+
+    private static Resource localResource(String path) {
+        return Resource.file(path, TestDatastoresTest.class.getClassLoader());
+    }
 }