Do not modify cache's CMAKE_CXX_FLAGS_RELEASE when configuring Ceres

Otherwise you'll have infinite appending of Ceres-specific flags
on every saving of any CmakeLists.txt.
Re-enable tests for multiview and image

For as long code is in repo and used by some tools
better to have it covered by tests.

Some of them are failing tho, but that's completely
different story to be addressed later.