test: Relax a debuginfo test

This test is blocking a snapshot. Apparently the snapshot bot doesn't print
'limited-debuginfo::main()' but rather just 'main()'. Who knew?
auto merge of #12839 : alexcrichton/rust/fix-snap, r=huonw

This test is blocking a snapshot. Apparently the snapshot bot doesn't print
'limited-debuginfo::main()' but rather just 'main()'. Who knew?