bpo-18578: Rename and document test.bytecode_helper as test.support.bytecode_helper (GH-15168)

Rename and document test.bytecode_helper as test.support.bytecode_helper
diff --git a/Lib/test/test_dis.py b/Lib/test/test_dis.py
index 11f97e6..fecf203 100644
--- a/Lib/test/test_dis.py
+++ b/Lib/test/test_dis.py
@@ -1,7 +1,7 @@
 # Minimal tests for dis module
 
 from test.support import captured_stdout
-from test.bytecode_helper import BytecodeTestCase
+from test.support.bytecode_helper import BytecodeTestCase
 import unittest
 import sys
 import dis