Fix: remove dead #undef __MPI toggles from source_base tests

`source/source_base/test/CMakeLists.txt:1` calls
`abacus_disable_feature_definitions(__MPI)`, so `__MPI` is never defined for any
target in that directory and every `#ifdef __MPI` in it is always false.

`opt_tn_test.cpp` and gave those two targets a real `mpi_test_main.cpp`. What
remains of the pattern is:

- `global_function_test.cpp` -- three `#ifdef __MPI / #undef __MPI / #endif`
  blocks. Their test bodies sit *outside* the `#ifdef`, so they always compiled;
  the blocks are pure no-ops.
- `math_chebyshev_test.cpp` -- guards wrapping `checkconverge` and
  `checkconverge_float`, so those two bodies were compiled out in *every*
  configuration: two tests reporting PASS while asserting nothing.
- `opt_cg_test.cpp`, `opt_test_tools.cpp` -- a file-level `#undef __MPI` that only
  duplicates what CMake already does.

The `#undef __MPI` / `#define __MPI` pairs were no-ops in any case: the
preprocessor cannot retroactively change headers already processed above, and the
library these tests link against is a separate translation unit, compiled with
`__MPI` regardless of what the test does.

The two chebyshev bodies this exposes are handled in the next commit.

Author: zxy.monado

source/source_base/test/global_function_test.cpp

@@ -695,9 +695,6 @@ TEST_F(GlobalFunctionTest,MemAvailable)
 
 TEST_F(GlobalFunctionTest,BlockHere)
 {
-#ifdef __MPI
-#undef __MPI
-#endif
 	std::string output2;
 	std::string block_in="111";
 	GlobalV::MY_RANK=1;
@@ -712,9 +709,6 @@ TEST_F(GlobalFunctionTest,BlockHere)
 
 TEST_F(GlobalFunctionTest,BlockHere2)
 {
-#ifdef __MPI
-#undef __MPI
-#endif
 	std::string output2;
 	std::string block_in="111";
 	GlobalV::MY_RANK=0;
@@ -733,9 +727,6 @@ TEST_F(GlobalFunctionTest,BlockHere2)
 
 TEST_F(GlobalFunctionTest,BlockHere3)
 {
-#ifdef __MPI
-#undef __MPI
-#endif
 	std::string output2;
 	std::string block_in="111";
 	GlobalV::MY_RANK=0;

source/source_base/test/math_chebyshev_test.cpp

@@ -333,8 +333,6 @@ TEST_F(MathChebyshevTest, tracepolyA)
 
 TEST_F(MathChebyshevTest, checkconverge)
 {
-#ifdef __MPI
-#undef __MPI
     const int norder = 100;
     p_chetest = new ModuleBase::Chebyshev<double>(norder);
     auto fun_sigma_y
@@ -376,8 +374,6 @@ TEST_F(MathChebyshevTest, checkconverge)
 
     delete[] v;
     delete p_chetest;
-#define __MPI
-#endif
 }
 
 TEST_F(MathChebyshevTest, recurs)
@@ -622,8 +618,6 @@ TEST_F(MathChebyshevTest, tracepolyA_float)
 
 TEST_F(MathChebyshevTest, checkconverge_float)
 {
-    #ifdef __MPI
-    #undef __MPI
     const int norder = 100;
     p_fchetest = new ModuleBase::Chebyshev<float>(norder);
 
@@ -647,6 +641,5 @@ TEST_F(MathChebyshevTest, checkconverge_float)
 
     delete[] v;
     delete p_fchetest;
-    #endif
 }
 #endif

source/source_base/test/opt_cg_test.cpp

@@ -1,6 +1,3 @@
-#ifdef __MPI
-#undef __MPI
-#endif
 #include "gtest/gtest.h"
 #include "../opt_cg.h"
 #include "../opt_dcsrch.h"

source/source_base/test/opt_test_tools.cpp

@@ -1,6 +1,3 @@
-#ifdef __MPI
-#undef __MPI
-#endif
 #include "./opt_test_tools.h"
 #include <math.h>