strfmon: Add tests for Austin Group Defect 1199

[jlduran]

Summary

Add tests for The Open Group Base Specifications Issue 8[1], Austin
Group Defect 1199[2].

Items marked with XXX represent an invalid output. These items will be
fixed in subsequent commits.

Notice that an existing test is now considered invalid.

Our locale definitions do not include int_p_sep_by_space nor
int_n_sep_by_space[3]. Those will be addressed in a subsequent commit.
However, the CLDR project defines them as "0", which causes the output
to appear as "USD123.45". If our locale definitions were to set the
international {n,p}_sep_by_space to "1", the output would display as the
expected "USD 123.45".

While here, use the SPDX license identifier and add my name to the
file.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/strfmon.html
[2]: https://www.austingroupbugs.net/view.php?id=1199
[3]: https://unicode-org.atlassian.net/browse/CLDR-237

[kib]

Would you add a link to the actual defect page? Is it public?

lib/libc/tests/stdlib/strfmon_test.c

15

Should be ordered alphabetically

[jlduran]

Address style(9) suggestions:

• Sort includes