tests/struct*: Make skippable.

tests/basics/struct2.py

@@ -3,7 +3,12 @@
 try:
     import ustruct as struct
 except:
-    import struct
+    try:
+        import struct
+    except ImportError:
+        import sys
+        print("SKIP")
+        sys.exit()
 
 print(struct.calcsize('0s'))
 print(struct.unpack('0s', b''))