ci: Fail if tests are not detected by ctest

CI runs pipeline in two jobs - build and test. If artifacts are not
restored properly, ctest will not be able to find the tests. Absence of
test by default is success in ctest. To make sure there is no error with
restoring workspace and hence ctest missing the test, fail if tests are
not detected by ctest.

Signed-off-by: Saheer Babu <saheer.babu@arm.com>