Support testing C integration tests

[ubaidsk]

From https://github.com/lfortran/lfortran-conda-testing/actions/runs/6449417776,

CMakeFiles/stop_01.dir/stop_01.f90.o.tmp.c:7:10: fatal error: lfortran_intrinsics.h: No such file or directory
    7 | #include <lfortran_intrinsics.h>
      |          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
The command 'gcc  -I/home/runner/micromamba/envs/lf/bin/../share/lpython/lib/impure -o CMakeFiles/stop_01.dir/stop_01.f90.o -c CMakeFiles/stop_01.dir/stop_01.f90.o.tmp.c' failed.
make[2]: *** [CMakeFiles/stop_01.dir/build.make:75: CMakeFiles/stop_01.dir/stop_01.f90.o] Error 11
make[1]: *** [CMakeFiles/Makefile2:253: CMakeFiles/stop_01.dir/all] Error 2