Terminate test on illegal instruction

This gets the CI going again, but we will want to fix the test
harness since it's useful to be able to debug the core after it
executes an illegal instruction.

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>

core.vhdl

@@ -271,6 +271,7 @@ begin
 		loop_0: for i in 0 to 31 loop
 		    report "REG " & to_hstring(registers(i));
 		end loop loop_0;
+		assert false report "end of test" severity failure;
 	    end if;
 	end process;
     end generate;