Convert the msun tests to ATF

This provides better error messages that just an assertion failure and
also makes it easier to mark individual tests as XFAIL.
It was also helpful when coming up with D28786 and D28787.

Differential Revision: https://reviews.freebsd.org/D28798

(cherry picked from commit 133bc645077d77ddb3c7d49130b4ec6fd93e7d34)