Exclude `n_slices.txt` from comparison

Because `$tests_old_dir/Extraction_eddies_region_5/SHPC` is opened
with `SHPC_class` in test `Plot_eddy_contours_window`, which will
create `n_slices.txt`, so there will always be this file only in
`$tests_old_dir/Extraction_eddies_region_5/SHPC`.

Inst_eddies/Tests/long_tests.json

@@ -72,7 +72,8 @@
 	],
 	"create_file": ["config_nml.txt", "&config_nml /\n"],
 	"input": "&dates_nml date = 20454/\n",
-	"description": "Same as Extraction_eddies_region_4 with larger domain."
+	"description": "Same as Extraction_eddies_region_4 with larger domain.",
+	"exclude_cmp": ["n_slices.txt"]
     },
     {
         "create_file": ["config_nml.txt", "&config_nml /\n"],