tests: Rename test scripts, changing - to _ for consistency.

From now on, all new tests must use underscore.

Addresses issue #727.

tests/basics/with_return.py

@@ -0,0 +1,14 @@
+class CtxMgr:
+
+    def __enter__(self):
+        print("__enter__")
+        return self
+
+    def __exit__(self, a, b, c):
+        print("__exit__", repr(a), repr(b))
+
+def foo():
+    with CtxMgr():
+        return 4
+
+print(foo())