test_inf_inputs: Use atf_tc_expect_fail() instead of atf_tc_skip()

Reviewed By: lwhsu
Differential Revision: https://reviews.freebsd.org/D28396

(cherry picked from commit 4d2edf3af1dbd8a3e7cf1b22343a1ecfc2dd41ba)

Fix lib/msun's ctrig_test/test_inf_inputs test case with clang >= 10

This sprinkles a few strategic volatiles in an attempt to defeat clang's
optimization interfering with the expected floating-point exception
flags.

Reported by: lwhsu
PR: 244732

(cherry picked from commit ac76bc1145dd7f4476e5d982ce8f355f71015713)