atime unit test: do not compare input file's current atime

and explain why (missing O_NOATIME open mode modified the atime at backup time).

borg/testsuite/archiver.py

@@ -297,7 +297,10 @@ class ArchiverTestCase(ArchiverTestCaseBase):
         sti = os.stat('input/file1')
         sto = os.stat('output/input/file1')
         assert st_mtime_ns(sti) == st_mtime_ns(sto) == mtime * 1e9
-        assert st_atime_ns(sti) == st_atime_ns(sto) == atime * 1e9
+        # to support testing on platforms without O_NOATIME open mode, we just
+        # compare the restored file with the original atime (not with the
+        # atime it has now).
+        assert st_atime_ns(sto) == atime * 1e9
 
     def _extract_repository_id(self, path):
         return Repository(self.repository_path).id