Move tests test_verify_checksum_* to separate class

Moved tests in virt.libvirt.test_imagecache.test_verify_checksum_*
to separate class.

Removed _intercept_log_messages() from ImageCacheManagerTestCase
and VerifyChecksumTestCase classes and turned it into a
module method.

This commit is the first step in refactoring the file.
It is followed by the commit which removes duplicated code.

bp nova-tests-code-duplication

Change-Id: Ie5bffcabb83ec2208ced343cdf75bf1197b7d5c3
Merge "Move tests test_verify_checksum_* to separate class"