image-buildinfo,oeqa/selftest/containerimage: Ensure image-buildinfo doesn't break...

image-buildinfo,oeqa/selftest/containerimage: Ensure image-buildinfo doesn't break tests

Having image-buildinfo enabled causes containerimage.ContainerImageTests.test_expected_files
to fail due to the presence of an unexpected file:
  ['./',
   './etc/',
-  './etc/build',
   './etc/default/',
   './etc/default/postinst',

Tweak the class to allow it to be disabled and disable it from the test just in
case it was enabled.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>