Avoid double output in fenv_test

This tests fork()s, so if there is still data in the stdout buffer on fork
it will print it again in the child process. This was happening in the
CheriBSD CI and caused the test to complain about malformed TAP output.

Reviewed By: ngie
Differential Revision: https://reviews.freebsd.org/D28397

(cherry picked from commit 892a05ee3a6abd8d84f46a1b895b1d2bf805d08a)