tests/qemu-iotests: Remove the "_supported_os Linux" line from many tests

tests/qemu-iotests: Remove the "_supported_os Linux" line from many tests

A lot of tests run fine on FreeBSD and macOS, too - the limitation
to Linux here was likely just copied-and-pasted from other tests.
Thus remove the "_supported_os Linux" line from tests that run
successful in our CI pipelines on FreeBSD and macOS.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20190502084506.8009-6-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>