Decl'HDF5 always depends on MPI for tests

even when building against a sequential HDF5, Decl'HDF5 depends on MPI for tests.

cmake -DCMAKE_INSTALL_PREFIX=/usr -DCMAKE_BUILD_TYPE=None -DCMAKE_INSTALL_SYSCONFDIR=/etc -DCMAKE_INSTALL_LOCALSTATEDIR=/var -DCMAKE_EXPORT_NO_PACKAGE_REGISTRY=ON -DCMAKE_FIND_PACKAGE_NO_PACKAGE_REGISTRY=ON "-GUnix Makefiles" -DCMAKE_VERBOSE_MAKEFILE=ON -DCMAKE_INSTALL_LIBDIR=lib/x86_64-linux-gnu -DBUILD_HDF5_PARALLEL=OFF -DBUILD_TESTING=ON -DHDF5_C_COMPILER_EXECUTABLE=/usr/bin/h5cc -DINSTALL_PDIPLUGINDIR=/usr/lib/x86_64-linux-gnu/pdi/plugins_1.2.1/serial -DCMAKE_BUILD_TYPE=Release -DMPI_C_COMPILER=mpicc.serial -DMPI_CXX_COMPILER=mpicxx.serial -DMPI_Fortran_COMPILER=mpif90.serial -DMPIEXEC_EXECUTABLE=/usr/bin/mpiexec.serial ../plugins/decl_hdf5
-- The C compiler identification is GNU 10.2.1
-- The CXX compiler identification is GNU 10.2.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- The Fortran compiler identification is GNU 10.2.1
-- Detecting Fortran compiler ABI info
-- Detecting Fortran compiler ABI info - done
-- Check for working Fortran compiler: /usr/bin/gfortran - skipped
-- Checking whether /usr/bin/gfortran supports Fortran 90
-- Checking whether /usr/bin/gfortran supports Fortran 90 - yes
-- Could NOT find MPI_C (missing: MPI_C_LIB_NAMES MPI_C_HEADER_DIR MPI_C_WORKS) 
-- Could NOT find MPI_CXX (missing: MPI_CXX_LIB_NAMES MPI_CXX_HEADER_DIR MPI_CXX_WORKS) 
-- Could NOT find MPI_Fortran (missing: MPI_Fortran_LIB_NAMES MPI_Fortran_F77_HEADER_DIR MPI_Fortran_MODULE_DIR MPI_Fortran_WORKS) 
CMake Error at cmake/FindPackageHandleStandardArgs.cmake:137 (message):
  Could NOT find MPI (missing: MPI_C_FOUND MPI_CXX_FOUND MPI_Fortran_FOUND
  CXX C Fortran)
Call Stack (most recent call first):
  cmake/FindPackageHandleStandardArgs.cmake:378 (_FPHSA_FAILURE_MESSAGE)
  cmake/FindMPI.cmake:1461 (find_package_handle_standard_args)
  CMakeLists.txt:58 (find_package)