Updated (final?) patch. This corrects the internals of the gzip extra field in one of the test files (not relevant to the test, but matter of spec-compliance), changes a native-libs-only assertion to a warning to work around Apache Hudson's lack of support, and removes all but three commented-out code blocks.
The three remaining blocks are all a result of other bugs: the aforementioned Hudson native-library one (lack of coverage of a key infrastructure component is a problem, and it's getting worse with every new native codec);
HADOOP-6799 (broken reset() on gzip compression); and HADOOP-6852 (broken bzip2 decompression, apparently). I realize commented-out code is frowned upon as a style issue, but I would rather not remove useful tests that are currently blocked by things outside my (immediate) control. I'll defer to committer's opinion, however.
This passed "ant test-patch" on a 10-node cluster, so I'm going to "submit patch" to see if it gets past Apache's Hudson, too.