CI: Use TESTIMAGE_AUTO

Now that TESTIMAGE_AUTO is available for Zephyr builds, enable it in
ci/testimage.yml and remove the redundant build_and_test base
configuration.

Remove testimage from Nios2 build as it is currently failing.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Signed-off-by: Naveen Saini <naveen.kumar.saini@intel.com>