CMake: error out in certain case (kokkos#2115)

Graph unit tests are unique in that they use default_scalar for the
KokkosKernelsHandle. So if test-eti-only is ON, but neither float nor
double is instatiated, then error out for the graph unit tests.

Users can still build without float or double if they want, but only if
they turn off tests or the graph component.

graph/unit_test/CMakeLists.txt

@@ -10,6 +10,12 @@ KOKKOSKERNELS_INCLUDE_DIRECTORIES(REQUIRED_DURING_INSTALLATION_TESTING ${CMAKE_C
 #                   #
 #####################
 
+IF (KokkosKernels_TEST_ETI_ONLY)
+  IF (NOT KokkosKernels_INST_DOUBLE AND NOT KokkosKernels_INST_FLOAT)
+    MESSAGE(FATAL_ERROR "Because only ETI'd type combinations are enabled for testing, the Kokkos Kernels graph tests require that double or float is enabled in ETI.")
+  ENDIF ()
+ENDIF ()
+
 #####################
 #                   #
 # Add GPU backends  #