Improve test coverage

[tukusejssirs]

We should improve test coverage of romcal@3.

IMO we should first define the structure of tests, what should be tested and how we should test it.

IMHO we should test the following [WIP]:

• unit tests:
  • test all functions and methods defined in all files under lib/;
• e2e tests:
  • build romcal with different local timezones:
    • build romcal with local timezone;
    • build romcal with American timezone;
    • build romcal with European timezone;
    • build romcal with Asian timezone;

IMHO we should test romcal (AKA test rules):

• against a larger range of years:
  • we already do that, albeit partially;
  • we could test romcal since 1970 to 2100;
• we should try to find all possibilities (on which week day a particular celebration could occur, occurrances of different celebrations on the same day, etc) and test them all;
• never combine unrelated tests into test suites:
  • IMHO currently, we combine tests without any logic a bit;
• use Jest snapshots (or something else), to compare data of full generated years (a set of different years with a set of different calendars);