Ensuring a test uses a codec supporting DocValues

[Smiley, David W.]

I wrote a test that requires DocValues. It failed on me once because of the 
Codec randomization chose Lucene3x which doesn’t support DocValues.  What’s the 
best way to adjust my test to assure this doesn’t happen?

What I ended up doing was this:
indexWriterConfig.setCodec( _TestUtil.alwaysDocValuesFormat(new 
Lucene45DocValuesFormat()));
But I don’t like that I hard-coded a particular format.
(FYI the source file is an abstract base test class: SpatialTestCase, method 
newIndexWriterConfig )

Another approach might be to call:
assumeTrue(defaultCodecSupportsDocValues())
Although sometimes the test of course won’t be run at all instead of it 
preferably forcing a compatible format.

Thoughts?

~ David