Install mg test executables (rapidsai#4656)

`rapids_test_add()` does not install the executables we need, because it's checking the value of `${MPIEXEC_EXECUTABLE}` and seeing that it's not a target. Install the target separately.

Authors:
  - Kyle Edwards (https://github.com/KyleFromNVIDIA)

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - Chuck Hastings (https://github.com/ChuckHastings)

URL: rapidsai#4656

cpp/tests/CMakeLists.txt

@@ -220,6 +220,7 @@ function(ConfigureTestMG CMAKE_TEST_NAME)
         GPUS ${GPU_COUNT}
         PERCENT 100
         INSTALL_COMPONENT_SET testing_mg
+        INSTALL_TARGET ${CMAKE_TEST_NAME}
     )
     set_tests_properties(${CMAKE_TEST_NAME} PROPERTIES LABELS "CUGRAPH_MG")
 
@@ -302,6 +303,7 @@ function(ConfigureCTestMG CMAKE_TEST_NAME)
         GPUS ${GPU_COUNT}
         PERCENT 100
         INSTALL_COMPONENT_SET testing_mg
+        INSTALL_TARGET ${CMAKE_TEST_NAME}
     )
     set_tests_properties(${CMAKE_TEST_NAME} PROPERTIES LABELS "CUGRAPH_C_MG")