test-backend: Clean up leak data import files after test-suite run.

This is a simple, non-intrusive way of removing the bulk of the
clutter from `var/<uuid>/test-backend` after running `test-backend`.

Ideally, we'll replace this logic with proper tearDown methods.

tools/test-backend

@@ -16,6 +16,7 @@ import ujson
 import httplib2
 import httpretty
 import requests
+import shutil
 
 import django
 from django.conf import settings
@@ -456,6 +457,21 @@ def main() -> None:
         # an important clue as to why tests fail.
         report_slow_tests()
 
+    # We now cleanup files leaked by certain tests that don't clean up
+    # after themselves.  Ideally, this cleanup would happen in in the
+    # tearDown() methods for the relevant test classes, but this at least
+    # prevents a disk leak.
+    from scripts.lib.zulip_tools import get_or_create_dev_uuid_var_path
+    for path in glob.glob(os.path.join(get_or_create_dev_uuid_var_path('test-backend'),
+                                       "test-*-import-*")):
+        try:
+            if os.path.isdir(path):
+                shutil.rmtree(path)
+            else:
+                os.remove(path)
+        except FileNotFoundError:
+            pass
+
     # Ideally, we'd check for any leaked test databases here;
     # but that needs some hackery with database names.
     #