Fix typo in class name
test_helper: Raise a RuntimeError if run_cmd fails
test: Move image names into common module
test: NFC use 'with' to open file
tests: Don't print log messages during tests
tests: Revert to creating raw test images
tests: Connect to qemu:///system explicitly
tests: NFC build_tdl() returns image path
tests: Pass image name to TestImage()
tests: Create qcow2 overlay images for use in tests

Remove TestImage.open(): we add drives immediately on creation
Remove TestImage.close(): we rely on garbage collection to close
tests: Fix template tdl creation

Replacing TDL_DIR didn't strip the last '/' from the template name, and
os.path.join() won't join an absolute path.
tests: NFC Use with for implicit close()
tests: NFC Indentation fix
tests: Don't automatically generate test images

Test images are now generated explicitly by running test_helper.py.