msun: ctanh/ctanhf: Import fix from MUSL

[arichardson]

Summary

This applies MUSL commit b02eed9c4841913d690a2d0029737d72615384fe by
Szabolcs Nagy and updates the tests accordingly. This also allows
removing an XFAIL from the test.

MUSL commit message:

complex: fix ctanh(+-0+i*nan) and ctanh(+-0+-i*inf)

These cases were incorrect in C11 as described by
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1886.htm

PR: 217528

[dim]

LGTM, I ran make check (with LD_LIBRARY_PATH set :) ) and it worked.