The race condition mentioned in the previous commit
"Stop CMake out of source tests running on 16.04"
has also been observed with test_cmake_as_subdirectory on 3.1 and can
presumably happen on 2.28 as well. So skip it on Ubuntu 16.04 as well.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>