strfmon: tests: Improve international currency test

[jlduran]

Summary

Slightly adapt some of the tests from the glibc project[1].

Note that we use monetary definitions from the Unicode Project, at the
moment, all differences in the output produced by strfmon(3) should be
related to the differences in locale definitions.

[1]: https://github.com/bminor/glibc/blob/e2b00d59eba07a97c10d540031fedc83df8b7f76/stdlib/tst-strfmon_l.c#L126-L220