gio/tests/converter-stream: add a new test, rename an old one

gio/tests/converter-stream: add a new test, rename an old one

Add a test that the decompressor input streams handle truncated data
correctly. (They do; I wrote the test thinking there was a bug there,
but there isn't.)

Also, rename the "corruption" tests to "roundtrip", since "corruption"
makes it sound like we're testing how the converters deal with
corrupted data, as opposed to merely testing that they don't corrupt
data themselves. And fix the bug reference.