YAML-based tests

[c960657]

I propose a new way to write tests for holiday providers.

Rather than testing each holiday separately, it may be easier to check all holidays in a given year for relevant years. This is particularly relevant for the current year +/- a few years, and for years in the past with various exceptions, such as the suspended holidays in Portugal 2013-2015, or substitute holidays, bridge holidays etc.

Listing all holidays for a given year makes it easy to check that all holidays (and no more) are returned for a given year, e.g. by comparing with other sources (example).

Also, some of the current tests tend to be a copy of the code they are testing (example). This is fine for regression testing, but such tests are less likely to detect flaws in the logic during initial implementation.

For this PR I propose YAML for storing the expected test data. The YAML files are a bit verbose (though not as verbose as e.g. JSON), but we could consider crafting our own simple format for this purpose.

The YAML tests can be auto-generated, lowering the barrier for contributing new holiday providers. With the current approach to testing, it requires a substantial amount of work to provide test cases. The YAML files provided for this PR were all auto-generated (except the few comment entries). Of course, if you prefer TDD, you can also hand-edit the YAML files first.

The full lists of holidays makes it easy to help with QA, even if you are not familiar with the Yasumi code.

For this PR, I have added full coverage for at least one locale for all calendar providers for all years in the range 2010-2030. In addition to this, we should add years in the past of specific interest (exceptions, substitute holidays etc.). We could easily add more years – I don't have a strong opinion about this.

I have allowed the possibility to add comments to the YAML file, either for a specific holiday (example), or the entire year (entries with no dateand name entries but only a comment entry – example). This is a way to mention things to pay attention to, if the files are later changed (example).

If we adopt this approach, I think the YAML files can eliminate the need for some of the existing tests. However, I think the possible removal/adjustment of these should be a discussed in a separate PR.

For now I have just included all years 2010-2030. If you like this proposal, I will extend the YAML file with more relevant years for each specific provider.

What do you think?

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[c960657]

As an alternative to YAML, we could use Markdown. The source data is even more compact, and it is very review-friendly, because Github automatically formats Markdown.

Example:

# 2012
Date       | Type     | Name                | Comment
---------- | -------- | ------------------- | ---
2012-01-02 | bank     | New Year's Day
2012-04-06 | official | Good Friday
2012-04-09 | bank     | Easter Monday
2012-05-07 | bank     | May Day Bank Holiday
2012-06-04 | bank     | Spring Bank Holiday | Moved to 4 June for the celebration of the Diamond Jubilee of Elizabeth II.
2012-08-27 | bank     | Summer Bank Holiday
2012-12-25 | official | Christmas Day
2012-12-26 | bank     | Boxing Day

# 2013
...

2012

Date	Type	Name	Comment
2012-01-02	bank	New Year's Day
2012-04-06	official	Good Friday
2012-04-09	bank	Easter Monday
2012-05-07	bank	May Day Bank Holiday
2012-06-04	bank	Spring Bank Holiday	Moved to 4 June for the celebration of the Diamond Jubilee of Elizabeth II.
2012-08-27	bank	Summer Bank Holiday
2012-12-25	official	Christmas Day
2012-12-26	bank	Boxing Day