Force flush of stdout in new-testing for each line

In order to allow to watch live testing results, with tools like less +F.

If flush isn't forced, results are written in slurm output file only by blocks of many lines, not really practical to watch the progression of the tests.

new-testing/run_list.c

@@ -927,5 +927,6 @@ run_print_line( const testing_t *test, const run_arg_list_t *arglist,
     }
 
     fprintf( stdout, "%s\n", str );
+    fflush(stdout);
     return;
 }