Enforce deterministic ordering in file walk.

os.walk internally calls os.scandir, whose "entries are yielded in
arbitrary order."  This breaks (at least on my machine) two unit tests
which rely on ordering:

1. test_reprocessing.test_log_collection_tgen
2. test_reprocessing.test_log_collection_torctl

This patch fixes the problem by calling sorted on the arbitrarily
ordered file names.

onionperf/reprocessing.py

@@ -13,7 +13,7 @@
 def collect_logs(dirpath, pattern):
     logs = []
     for root, dirnames, filenames in os.walk(dirpath):
-        for filename in fnmatch.filter(filenames, pattern):
+        for filename in fnmatch.filter(sorted(filenames), pattern):
             logs.append(os.path.join(root, filename))
     return logs