Change CMakeFiles to find the correct tests directory

[g7fernandes]

It tries to look into src/tests, but the tests are now located at ./tests.

Consequence:

CMake Error at CMakeLists.txt:147 (add_executable):
  Cannot find source file:

    lennard_jones/build/FCSV-source/src/tests/csv_test.f90

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm .h
  .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90 .f95 .f03 .hip .ispc


CMake Error at CMakeLists.txt:147 (add_executable):
  No SOURCES given to target: csv_test