Add tests for "Fallback symbols displayed in formatted currency output when no currency is set in API" and for "Rounding behavior when no currency is set in API"

This behavior difference was discovered between ICU4C and ICU4J.

Sample code, ICU4C:

ICU4J:

Consensus opinion: we should use the unknown currency "XXX", like ICU4J and JDK are already doing in the ISO code format, with the following output:

The plural long-name string, "unknown currency", comes from locale data.

Follow up with CLDR to add the currency symbol.

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In addition, one more behavior difference was discovered.

ICU4C sample code:

ICU4J:

Consensus opinion: Since we should apply the default currency when no currency is set, as stated in row 10, we should also apply XXX's rounding rules, which are a fixed two fraction digits.

Desired output: