rasa test core not working for stories in a folder
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**Rasa version**:
Current ```master```
**Rasa SDK version** (if used & relevant):
N/A
**Rasa X version** (if used & relevant):
N/A
**Python version**:

**Operating system** (windows, osx, ...):

**Issue**:
`rasa test core` doesn't work when the stories are in a subfolder, e.g., `rasa test core --stories data/core` on `rasa-demo`
In this case, it finds 0 test stories. 

(My suspicion is that it comes from `rasa.data.get_test_directory`)

**Error (including full traceback)**:
```

```

**Command or request that led to error**:
```
rasa test core --stories data/core
```

in ```rasa-demo```

**Content of configuration file (config.yml)** (if relevant):
```yml

```

**Content of domain file (domain.yml)** (if relevant):
```yml

```

diff --git a/rasa/cli/test.py b/rasa/cli/test.py
index d866c3ae392..8858ed7d793 100644
--- a/rasa/cli/test.py
+++ b/rasa/cli/test.py
@@ -70,7 +70,11 @@ def run_core_test(args: argparse.Namespace) -> None:
     stories = rasa.cli.utils.get_validated_path(
         args.stories, "stories", DEFAULT_DATA_PATH
     )
-    stories = rasa.shared.data.get_test_directory(stories)
+    if args.e2e:
+        stories = rasa.shared.data.get_test_directory(stories)
+    else:
+        stories = rasa.shared.data.get_core_directory(stories)
+
     output = args.out or DEFAULT_RESULTS_PATH
     args.errors = not args.no_errors
 
diff --git a/rasa/shared/data.py b/rasa/shared/data.py
index 7b30c100733..8872943b7dc 100644
--- a/rasa/shared/data.py
+++ b/rasa/shared/data.py
@@ -238,7 +238,7 @@ def is_test_stories_file(file_path: Text) -> bool:
         MarkdownStoryReader,
     )
 
-    return YAMLStoryReader.is_yaml_story_file(
+    return YAMLStoryReader.is_yaml_test_stories_file(
         file_path
     ) or MarkdownStoryReader.is_markdown_test_stories_file(file_path)
 
['rasa/cli/test.py', 'rasa/shared/data.py']