tests/qemu-iotests: Don't use 'seq' in the iotests

The 'seq' command is not available by default on OpenBSD, so these
iotests are currently failing there. It could be installed as 'gseq'
from the coreutils package - but since it is using a different name
there and we are running the iotests with the "bash" shell anyway,
let's simply use the built-in double parentheses for the for-loops
instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190723111201.1926-1-thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>