Improve reliability of converter test error check

**Describe the bug**
Right now the criteria is whether the standard error file was empty or not.
On Cray machine, the MPI runner may print additional things. I think when OpenMPI is enabled with GPU, additional printout also tricked the testing.
So I feel it is better to change convert4qmc to print a completion message like QMCPACK to signal a healthy run.
The pass criteria is to see that specific line and return error code 0 from the converter run.

**Expected behavior**
Make testing easy.