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

From now on, all new tests must use underscore.

Addresses issue #727.

tests/basics/del_deref.py

@@ -0,0 +1,22 @@
+def f():
+    x = 1
+    y = 2
+    def g():
+        nonlocal x
+        print(y)
+        try:
+            print(x)
+        except NameError:
+            print("NameError")
+    def h():
+        nonlocal x
+        print(y)
+        try:
+            del x
+        except NameError:
+            print("NameError")
+    print(x, y)
+    del x
+    g()
+    h()
+f()