Use the env keyword of `test_compare.py`

Overlap/Tests/long_tests.json

@@ -2,11 +2,8 @@
     {
         "input": "&MAIN_NML max_delta = 4 /\n",
         "title": "EGg",
-        "stdout": "eddy_graph_stdout.txt",
+	"env": {"mpiexec": "$mpiexec"},
         "command":
-	[
-            "env", "mpiexec=$mpiexec", "$build_dir/Overlap/eddy_graph.sh", "1",
-            "$PWD/Inst_eddies_5_anti"
-        ]
+	["$build_dir/Overlap/eddy_graph.sh", "1", "$PWD/Inst_eddies_5_anti"]
     }
 ]