Return full path to avoid IsADirectoryError

When getting local path, the full path is not returned
only the directory (ADAPTER_DATA_DIR) raising an IsADirectoryError
when opening the CSP config file

Return the full path, directory + filename

Add test for get_local_path(filename) method

csp_billing_adapter_local/plugin.py

@@ -40,8 +40,7 @@ def get_local_path(filename):
     local_storage_path = Path(ADAPTER_DATA_DIR)
     if not local_storage_path.exists():
         local_storage_path.mkdir(parents=True, exist_ok=True)
-    local_storage_path.joinpath(filename)
-    return local_storage_path
+    return local_storage_path.joinpath(filename)
 
 
 @csp_billing_adapter.hookimpl(trylast=True)

tests/unit/test_plugin.py

@@ -31,6 +31,7 @@
 
 
 from csp_billing_adapter_local.plugin import (
+    get_local_path,
     get_cache,
     get_csp_config,
     update_cache,
@@ -47,6 +48,12 @@
     )
 
 
+def test_local_get_local_path():
+    """Test get_local_path(filename) in local plugin"""
+    expected_path = Path('/var/lib/csp-billing-adapter/foo')
+    assert get_local_path('foo') == expected_path
+
+
 def test_local_get_cache():
     """Test get_cache() in local plugin"""
     with patch(