test: fix flakey test case for issue 4388
test: fix flakey test case for issue 4388

Seems like we need to drag the stack for <autogenerated>:1 on Plan 9.

See http://build.golang.org/log/283b996102b833dd81c58301d78aceaa4fe9838b.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/95390043

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