tests: Use uppercase label for floppy in mkimageTests

Commit 10c1c41fd8e in dosfstools: "Write uppercase letters in label."
That commit has been in dosfstools since version 3.0.16, and Fedora 19
uses version 3.0.20.

For quick reference(copied from dosfstools commit message):
  * https://bugzilla.novell.com/show_bug.cgi?id=657011#c4
  * http://support.microsoft.com/kb/71715/en-us

Change-Id: Iffcf1c1cfbb324330af13435bce2e0f991e84afe
Signed-off-by: huntxu <mhuntxu@gmail.com>
Reviewed-on: http://gerrit.ovirt.org/16472
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>