add basic tests of KnownApks mechanism

The tests in tests/run-tests do provide coverage for these cases, but it is
not explicit, but just comes from testing that the index file has not
changed.  These tests make it explicit what is being tested.

tests/common.TestCase

@@ -22,6 +22,7 @@ import textwrap
 import yaml
 import gzip
 from argparse import ArgumentParser
+from datetime import datetime, timezone
 from zipfile import BadZipFile, ZipFile
 from unittest import mock
 from pathlib import Path
@@ -2935,6 +2936,37 @@ class CommonTest(unittest.TestCase):
             [{'url': s}], fdroidserver.common.parse_mirrors_config(mirrors)
         )
 
+    def test_KnownApks_recordapk(self):
+        """Test that added dates are being fetched from the index.
+
+        There are more related tests in tests/run-tests.
+
+        """
+        now = datetime.now(timezone.utc)
+        knownapks = fdroidserver.common.KnownApks()
+        for apkName in knownapks.apks:
+            knownapks.recordapk(apkName, 'ignored', default_date=now)
+        for appid, added in knownapks.apks.values():
+            self.assertNotEqual(added, now)
+
+    def test_KnownApks_recordapk_new(self):
+        """Test that new added dates work, and are not replaced later.
+
+        There are more related tests in tests/run-tests.
+
+        """
+        now = datetime.now(timezone.utc)
+        knownapks = fdroidserver.common.KnownApks()
+        fake_apk = 'fake.apk'
+        knownapks.recordapk(fake_apk, 'ignored', default_date=now)
+        for apk, (appid, added) in knownapks.apks.items():
+            if apk == fake_apk:
+                self.assertEqual(added, now)
+            else:
+                self.assertNotEqual(added, now)
+        knownapks.recordapk(fake_apk, 'ignored', default_date=datetime.now(timezone.utc))
+        self.assertEqual(knownapks.apks[fake_apk][1], now)
+
 
 APKS_WITH_JAR_SIGNATURES = (
     (