iterate over all files in toml-test/tests/{valid,invalid}, including sub dirs (#427)

tests/test_api.py

diff --git a/tests/test_api.py b/tests/test_api.py
index ceb7ef4..9aecf38 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -52,11 +52,13 @@
 
 def test_valid_tests():
     valid_dir = "toml-test/tests/valid/"
-    for f in os.listdir(valid_dir):
-        if not f.endswith("toml"):
-            continue
-        with open(os.path.join(valid_dir, f)) as fh:
-            toml.dumps(toml.load(fh))
+    for path, sub_dirs, files in os.walk(valid_dir):
+        for name in files:
+            f = os.path.join(path, name)
+            if not f.endswith("toml"):
+                continue
+            with open(f) as fh:
+                toml.dumps(toml.load(fh))
 
 
 def test_circular_ref():
@@ -168,12 +170,14 @@
 
 def test_invalid_tests():
     invalid_dir = "toml-test/tests/invalid/"
-    for f in os.listdir(invalid_dir):
-        if not f.endswith("toml"):
-            continue
-        with pytest.raises(toml.TomlDecodeError):
-            with open(os.path.join(invalid_dir, f)) as fh:
-                toml.load(fh)
+    for path, sub_dirs, files in os.walk(invalid_dir):
+        for name in files:
+            f = os.path.join(path, name)
+            if not f.endswith("toml"):
+                continue
+            with pytest.raises(toml.TomlDecodeError):
+                with open(f) as fh:
+                    toml.load(fh)
 
 
 def test_exceptions():