tests: shell: Fix bogus testsuite failure with 250Hz

tests: shell: Fix bogus testsuite failure with 250Hz

Previous fix for HZ=100 was not sufficient, a kernel with HZ=250 rounds
the 10ms to 8ms it seems. Do as Lukas suggests and accept the occasional
input/output asymmetry instead of continuing the hide'n'seek game.

Fixes: c9c5b5f621c37 ("tests: shell: Fix bogus testsuite failure with 100Hz")
Suggested-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>