refactor(source_estate): fold charge_mpi_test into test/, drop test_mpi/

charge_mpi_test is a real MPI test (its main() calls MPI_Init) but does not
need a separate directory now that AddTest supports KEEP_FEATURE_DEFINITIONS.
Move it into test/ alongside the other module tests, keep __MPI for that
target, and register its mpirun -np 4 variant there. Remove the now-empty
test_mpi/ subdirectory and its add_subdirectory() call.

Verified: ctest -R "MODULE_ESTATE_charge_mpi_test|MODULE_ESTATE_test_rhog_io"
-> 3/3 passed (test_rhog_io 10/10, charge_mpi_test, charge_mpi_test_4np 4/4).

Author: abacus_fixer

source/source_estate/CMakeLists.txt

@@ -75,7 +75,6 @@ endif()
 if(BUILD_TESTING)
   if(ENABLE_MPI)
     add_subdirectory(test)
-    add_subdirectory(test_mpi)
   endif()
 endif()
 

source/source_estate/test/CMakeLists.txt

@@ -126,4 +126,18 @@ AddTest(
   KEEP_FEATURE_DEFINITIONS __MPI
 )
 
+AddTest(
+  TARGET MODULE_ESTATE_charge_mpi_test
+  LIBS parameter psi base device planewave
+  SOURCES charge_mpi_test.cpp ../module_charge/charge_mpi.cpp
+  # Real MPI test: its main() calls MPI_Init unconditionally, so it must keep
+  # __MPI despite this directory disabling it.
+  KEEP_FEATURE_DEFINITIONS __MPI
+)
+
+add_test(NAME MODULE_ESTATE_charge_mpi_test_4np
+      COMMAND mpirun -np 4 ./MODULE_ESTATE_charge_mpi_test;
+      WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}
+)
+
 endif()