Remove test case for ring modulator

Author: ivan-pi

example/CMakeLists.txt

@@ -1,5 +1,9 @@
-foreach(example robertson vanpol lorenz predator_prey pendant_drop three_equation_system oregonator e5_chemical hires ring_modulator)
+foreach(example robertson vanpol lorenz predator_prey pendant_drop three_equation_system oregonator e5_chemical hires)
   add_executable(${example} ${example}.f90)
   target_link_libraries(${example} PRIVATE stiff3)
   add_test(NAME ${example} COMMAND ${example})
 endforeach()
+
+# The test takes too long
+add_executable(ring_modulator ring_modulator.f90)
+target_link_libraries(ring_modulator PRIVATE stiff3)