test: Add a test for #7663

I think that the test case from this issue has become out of date with resolve
changes in the past 9 months, and it's not entirely clear to me what the
original bug was.

Regardless, it seems like tricky resolve behavior, so tests were added to make
sure things resolved correctly and warnings were correctly reported.

Closes #7663
auto merge of #13403 : alexcrichton/rust/issue-7663, r=brson

I think that the test case from this issue has become out of date with resolve
changes in the past 9 months, and it's not entirely clear to me what the
original bug was.

Regardless, it seems like tricky resolve behavior, so tests were added to make
sure things resolved correctly and warnings were correctly reported.

Closes #7663