qa: fix osd pool ls invalid test

expect_false does not extend past the pipe and fails because the command
succeeds

introduced in f05c977bbcd310a83e6df03344a72d4cc06e1cc3

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2622 from dachary/wip-osd-pool-ls

qa: fix osd pool ls invalid test