Running the out of source CMake test on Ubuntu 16.04 using more than one
processor (as the CI does) can create a race condition whereby the build
fails to see a generated file, despite that file actually having been
generated. This problem appears to go away with 18.04 or newer, so make
the out of source tests not supported on Ubuntu 16.04
Signed-off-by: Paul Elliott <paul.elliott@arm.com>