Add plural rules API tests

[echeran]

No description provided.

[sffc]

CLDR has plural rules tests in the form of samples in the plural rules files:

https://github.com/unicode-org/cldr/blob/c7e39f13da2bfbaf58c1447d610627511c7a6549/common/supplemental/plurals.xml

For example:

        <pluralRules locales="sl">
            <pluralRule count="one">v = 0 and i % 100 = 1 @integer 1, 101, 201, 301, 401, 501, 601, 701, 1001, …</pluralRule>
            <pluralRule count="two">v = 0 and i % 100 = 2 @integer 2, 102, 202, 302, 402, 502, 602, 702, 1002, …</pluralRule>
            <pluralRule count="few">v = 0 and i % 100 = 3..4 or v != 0 @integer 3, 4, 103, 104, 203, 204, 303, 304, 403, 404, 503, 504, 603, 604, 703, 704, 1003, … @decimal 0.0~1.5, 10.0, 100.0, 1000.0, 10000.0, 100000.0, 1000000.0, …</pluralRule>
            <pluralRule count="other"> @integer 0, 5~19, 100, 1000, 10000, 100000, 1000000, …</pluralRule>
        </pluralRules>

The things following @integer and @decimal are test cases.

[sven-oly]

We also need to handle "compact" format, indicated with a "c" instead of "e" in an exponential format.