[2.7] Rename test_ file that is really a support file to remove test_ prefix. (#2548)

[2.7] Rename test_ file that is really a support file to remove test_ prefix. (#2548)

I thought I had run the full test suite before the last checkin, but
obviously I didn't.  test_multibytecodec_support.py isn't really a test file,
it is a support file that contains a base test class.  Rename it to
multibytecodec_support so that regrtest test discovery doesn't think it is a
test file that should be run..
(cherry picked from commit 75d9aca97ad3c3d823d2f2211f579454b3216f51)