test_classes: Fix data import path reuse parallelism errors.

The make_import_output_dir helper function used a path determined
primarily by the filename of the fixture being used, and expected to
have complete control over that path for the duration of the test.

This resulted in nondeterministic errors if our two test classes that
ran Mattermost import code ran at the same time.

zerver/lib/test_classes.py

@@ -63,6 +63,7 @@ import re
 import ujson
 import urllib
 import shutil
+import tempfile
 
 API_KEYS = {}  # type: Dict[str, str]
 
@@ -692,8 +693,8 @@ class ZulipTestCase(TestCase):
             shutil.rmtree(path)
 
     def make_import_output_dir(self, exported_from: str) -> str:
-        output_dir = "var/test-{}-import".format(exported_from)
-        self.rm_tree(output_dir)
+        output_dir = tempfile.mkdtemp(dir="var/",
+                                      prefix="test-" + exported_from + "-import-")
         os.makedirs(output_dir, exist_ok=True)
         return output_dir