iotests: define group in each iotest

We are going to drop group file. Define group in tests as a preparatory
step.

The patch is generated by

    cd tests/qemu-iotests

    grep '^[0-9]\{3\} ' group | while read line; do
        file=$(awk '{print $1}' <<< "$line");
        groups=$(sed -e 's/^... //' <<< "$line");
        awk "NR==2{print \"# group: $groups\"}1" $file > tmp;
        cat tmp > $file;
    done

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210116134424.82867-7-vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
diff --git a/tests/qemu-iotests/261 b/tests/qemu-iotests/261
index 847b4c6..d1c8037 100755
--- a/tests/qemu-iotests/261
+++ b/tests/qemu-iotests/261
@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# group: rw
 #
 # Test case for qcow2's handling of extra data in snapshot table entries
 # (and more generally, how certain cases of broken snapshot tables are