summary.t test and single digit weeks

Hey @lauft I saw you applied a fix to the `summary.t` test to make sure it works properly in single digit weeks (1a6c30b93ae04406be6be4cb4f619b30f221fd7b). There is another interesting case, though, that happens even rarer:

Testing close to the year transition. In this case we have double digit and single digit weeks combined. I think we need to take care of this as well. Compare to this output, which happened during Ubuntu testing:
```
not ok 4 - summary.t: Summary should work with :all hint
# FAIL: AssertionError on file /tmp/autopkgtest.2VjSdd/autopkgtest_tmp/test/summary.t line 207 in test_with_all_hint: 'self.assertIn("""':
# '
# Wk Date Day ID Tags Start End Time Total
# --- ---------- --- -- ---- -------- -------- ------- -------
# W53 2021-01-03 Sun @3 FOO 10:00:00 11:00:00 1:00:00 1:00:00
# W1 2021-01-04 Mon @2 BAR 10:00:00 11:00:00 1:00:00 1:00:00
# W1 2021-01-05 Tue @1 BAZ 10:00:00 11:00:00 1:00:00 1:00:00
#
# 3:00:00
# ' not found in '
# Wk Date Day ID Tags Start End Time Total
# --- ---------- --- -- ---- -------- -------- ------- -------
# W53 2021-01-03 Sun @3 FOO 10:00:00 11:00:00 1:00:00 1:00:00
# W1 2021-01-04 Mon @2 BAR 10:00:00 11:00:00 1:00:00 1:00:00
# W1 2021-01-05 Tue @1 BAZ 10:00:00 11:00:00 1:00:00 1:00:00
#
# 3:00:00
#
# '
```
(https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-hirsute/hirsute/amd64/t/timew/20210104_035924_cff59@/log.gz)