Move from `Input/Region_4` to `Input`

`NSODS_in.txt` and `non_successive_overlap_in.txt` were the only files
left in directory `Region_4`. Not enough files to justify a directory.

Overlap/Tests/short_tests.json

@@ -253,7 +253,7 @@
         ]
     },
     {
-        "stdin_filename": "$src_dir/Overlap/Tests/Input/Region_4/non_successive_overlap_in.txt",
+        "stdin_filename": "$src_dir/Overlap/Tests/Input/non_successive_overlap_in.txt",
         "description": "Overlap of a snapshot with itself. Same as Successive_overlap, except for max_delta.",
         "title": "Non_successive_overlap",
         "stdout": "test_overlap_stdout.txt",
@@ -272,7 +272,7 @@
         ]
     },
     {
-        "stdin_filename": "$src_dir/Overlap/Tests/Input/Region_4/NSODS_in.txt",
+        "stdin_filename": "$src_dir/Overlap/Tests/Input/NSODS_in.txt",
         "description":
 	"Overlap of different snapshots. Same as Successive_overlap_different_snapshots, except for max_delta.",
         "title": "NSO_different_snapshots",