make it easy to keep test artifacts from jobs

When troubleshooting things that are difficult to reproduce locally, like
different behaviors in the fedora_latest job, these changes make it easy to
keep the test files around after the tests run.  For example, if PNGs are
processed differently by newer Python versions.

tests/nightly.TestCase

@@ -70,7 +70,10 @@ class NightlyTest(unittest.TestCase):
 
     def tearDown(self):
         self.tempdir.cleanup()
-        os.rmdir(self.testroot)
+        try:
+            os.rmdir(self.testroot)
+        except OSError:  # other test modules might have left stuff around
+            pass
 
     def _copy_test_debug_keystore(self):
         self.dot_android.mkdir()

tests/testcommon.py

@@ -18,6 +18,9 @@
 import os
 import sys
 import tempfile
+import unittest
+
+from pathlib import Path
 
 
 class TmpCwd:
@@ -60,3 +63,12 @@ def mkdtemp():
         return tempfile.TemporaryDirectory()
     else:
         return tempfile.TemporaryDirectory(ignore_cleanup_errors=True)
+
+
+def mkdir_testfiles(localmodule, test):
+    """Keep the test files in a labeled test dir for easy reference"""
+    testroot = Path(localmodule) / '.testfiles'
+    testroot.mkdir(exist_ok=True)
+    testdir = testroot / unittest.TestCase.id(test)
+    testdir.mkdir(exist_ok=True)
+    return tempfile.mkdtemp(dir=testdir)