Replace `stdin_filename` by input

As the namelist is now shorter, it is no longer convenient to write it
into a file.

Overlap/Tests/Input/write_overlap_delta_3_nml.txt

-&main_nml k = 4, delta = 3,
-COORD_EXTR_1(1) = 6.2,
-coord_extr_2 = 0.1, 1.,
-ssh_extr_2 = 1., m = 1 /

Overlap/Tests/short_tests.json

@@ -170,8 +170,8 @@
         "command": "$build_dir/Overlap/test_write_overlap"
     },
     {
-        "description": "Different coordinates and ssh, shifting in longitude, delta = 3.",
-        "stdin_filename": "$src_dir/Overlap/Tests/Input/write_overlap_delta_3_nml.txt",
+        "description": "delta = 3.",
+        "input": "&main_nml k = 4, delta = 3 /\n",
         "title": "Write_overlap_delta_3",
         "command": "$build_dir/Overlap/test_write_overlap"
     },