Add test for after_*() functions & missing after_archive_tarball()

[douglatornell]

New test inspects next_workers module to confirm that it contains an after_*() function
for each of the worker modules in nowcast/workers/.

Prompted by having missed the addition of the after_archive_tarball() function in PR#104.

[codecov]

Codecov Report

Merging #107 (40478f7) into main (bf9fddb) will decrease coverage by 0.00%.
The diff coverage is 70.58%.

@@            Coverage Diff             @@
##             main     #107      +/-   ##
==========================================
- Coverage   73.81%   73.81%   -0.01%     
==========================================
  Files         125      125              
  Lines       16249    16266      +17     
  Branches     1320     1326       +6     
==========================================
+ Hits        11995    12007      +12     
- Misses       4202     4205       +3     
- Partials       52       54       +2     

Flag	Coverage Δ
unittests	73.81% <70.58%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
tests/test_next_workers.py	99.03% <66.66%> (-0.69%)	⬇️
nowcast/next_workers.py	98.36% <100.00%> (+<0.01%)	⬆️

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