GZIPInputStream/GZIPOutputStream test improvements.

1. Ensure that there's a direct mapping from the class to its test,
so we're more likely to run the right tests. I've broken GzipTest
into two for this.

2. Include actual data rather than always round-tripping, to avoid
potential symmetric errors. (This wasn't important in this case.)

3. Remove a dead file that belonged to a test that's already been
removed.

Bug: 3164285
Change-Id: I312237454eead26d0dbbdb8f6339aa29ed8eea4e