tests: don't make tail's pipe-f2 test take the full 10 seconds

tests: don't make tail's pipe-f2 test take the full 10 seconds

* tests/tail-2/pipe-f2: Don't always wait 10 seconds.
Before, this test would always wait 10 seconds.
Now, it stops early when it detects that tail -f has written output.
BTW, the race condition that prompted changing the timeout from 1 second
to 10 was that tail -f could be killed by the timeout before producing
any output.