sh tests: Fix racy test11.0

This was sometimes exiting while the child fifo was created resulting in
[ENOTEMPTY] from rm. The child fifo isn't needed, just sleep.

PR: 290837
Fixes: e31fb97148f ("read builtin: Empty variables on timeout")
MFC after: 3 days

(cherry picked from commit 5d55553e160a93788372785300f5bd07c2451f93)