Optimized code structure

[jokervTv]

Dear, sir

Fixed linker error by restructuring the code when tried to use cmake to integrate doctest with mpi.

In origin file:

#ifdef DOCTEST_CONFIG_IMPLEMENT
// declarations and definitions, like
int mpi_comm_world_size();
// skip
int mpi_comm_world_size() {
// skip
}
#else
// only declarations
int mpi_comm_world_size(); // no definition
//skip
#define MPI_CHECK // define some macro for mpi test
// ...
#endif

So we have to define DOCTEST_CONFIG_IMPLEMENT to get the function definitions and get macro without DOCTEST_CONFIG_IMPLEMENT defined and then link them.
I don't think this approach is good enough for some tests, for example if there is only one file here.
That's what I think is wrong.

[codecov]

Codecov Report

Merging #680 (576172c) into dev (ae7a135) will not change coverage.
Report is 2 commits behind head on dev.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #680   +/-   ##
=======================================
  Coverage   92.11%   92.11%           
=======================================
  Files           2        2           
  Lines        2169     2169           
=======================================
  Hits         1998     1998           
  Misses        171      171