Extend base `common::TestCase` with category specific methods

[arkid15r]

Describe the solution you'd like

Extend entity test case with category specific methods to simplify test cases similar to https://github.com/vacanza/holidays/blob/72d70ef9c310b55de0fe9c6952fc439a4306d624/tests/countries/test_taiwan.py#L25

• update TestCase::setUpClass to have instances for each entity category (both observed/non-observed?)
• extend TestCase with a syntactic sugar methods that cover specific category tests (let's try avoid writing code for each separate category and use the approach similar to HolidayBase::_add_holiday_*, e.g. assertGovernmentHolidayName, assertOptionalNoNonObservedHoliday)

[SammanSarkar]

Hi, is your organization participating in GSoC 2025? I didn't see it on the GSoC website.

[SammanSarkar]

I would like to work on this. Please assign this to me @arkid15r