Fix doi_banner module and add tests

[GraemeWatt]

Closes #1005.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.82%. Comparing base (1f3e110) to head (f5f946f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1006      +/-   ##
==========================================
+ Coverage   86.65%   86.82%   +0.16%     
==========================================
  Files          59       59              
  Lines        5821     5820       -1     
==========================================
+ Hits         5044     5053       +9     
+ Misses        777      767      -10     

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[github-actions]

Coverage Report for CI Build 31098320747

Coverage increased (+0.2%) to 86.821%

Details

• Coverage increased (+0.2%) from the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	5820
Covered Lines:	5053
Line Coverage:	86.82%
Coverage Strength:	0.87 hits per line

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💛 - Coveralls