Add test coverage for 'today_fmt' reference substitution

[jayaddison]

Type of change

• Additional test coverage

Purpose

• During some self-review of Disable localisation when SOURCE_DATE_EPOCH is set #10949 using pytest, coverage and diff-cover, I noticed that one of the lines affected by the changeset that didn't have any test coverage is this code to substitute named references to 'today' in source documentation with a today_fmt-formatted date. This pull request offers basic test coverage for that part of the code.

Detail

• The DefaultSubstitution class is added as part of a chain of default transformers during app setup. It can substitute named references in a source document with configured text replacements, and has a special case to handle |today| when it appears as a reference with no direct text substitute (but instead a date format substitute) is configured.
• Indirectly this also adds test coverage for the handling of SOURCE_DATE_EPOCH within util.i18n.format_date, although that wasn't the initial intent of this changeset

Relates

• Disable localisation when SOURCE_DATE_EPOCH is set #10949