Plug MPI tests even when MPI is not used to build the code

This allows to test Messenger functionalities even for sequential builds

tests/CMakeLists.txt

@@ -13,12 +13,10 @@ add_executable (unit_tests
   test_TinyVector.cpp
   )
 
-if (${PASTIS_HAS_MPI})
 add_executable (mpi_unit_tests
   mpi_test_main.cpp
   mpi_test_Messenger.cpp
   )
-endif()
 
 target_include_directories(Catch2 INTERFACE ${CATCH_INCLUDE_DIR})
 
@@ -28,7 +26,6 @@ target_link_libraries (unit_tests
   Catch2
   )
 
-if (${PASTIS_HAS_MPI})
 target_link_libraries (mpi_unit_tests
   PastisUtils
   kokkos
@@ -36,7 +33,6 @@ if (${PASTIS_HAS_MPI})
   ${MPI_CXX_LINK_FLAGS} ${MPI_CXX_LIBRARIES}
   Catch2
   )
-endif()
 
 enable_testing()
 
@@ -48,9 +44,10 @@ if (${PASTIS_HAS_MPI})
     ${MPIEXEC_PREFLAGS}
     ${PASTIS_BINARY_DIR}/mpi_unit_tests
     ${MPIEXEC_POSTFLAGS})
+else()
+  add_test(mpi_unit_tests ${PASTIS_BINARY_DIR}/mpi_unit_tests)
+endif()
 
 # Should patch ParseAndAddCatchTests macro to allow additional command parameter
 # to launch tests
-  set_tests_properties("mpi_unit_tests" PROPERTIES FAIL_REGULAR_EXPRESSION "No tests ran"
-                                                    LABELS "mpi")
-endif()
+set_tests_properties("mpi_unit_tests" PROPERTIES FAIL_REGULAR_EXPRESSION "No tests ran" LABELS "mpi")