testsuite: Update #3601 test (it's still xfailed)
testsuite: Move comments, unbreak the build

Move comments to work around a pretty-printer bug. Arguably
it would be better to fix the bug, but I think it's just part of
the very long-standing issue with how the pretty-printer deals with
comments and line breaks.
auto merge of #5094 : catamorphism/rust/issue-3601-test, r=catamorphism