Remove redundant tests from Qcow2TestCase

Qcow2TestCase had exact copies of two tests in the parent
_ImageTestCase so remove them. I'm assuming the tests originated
in Qcow2TestCase but later more image types were added and the
tests were pulled up into the common parent mixin since they
applied to all types of images, but the redundant tests from
Qcow2TestCase weren't cleaned up.

Related-Bug: #1302211

Change-Id: I031516b555ead5a3769fb62d90e373aac1b6baf1
Merge "Remove redundant tests from Qcow2TestCase"