Enable half tests for copysign, ilogb, nextafter

http://b/23536224

Tests for copysign, ilogb and nextafter were disabled until they could
be added to the spec and headers.  This CL enables these tests.

Test for nextafter is updated so inputs in subnormal range are not
tested.

Change-Id: I27ca961de650362f0bb55fda36c449523773c8a2
1 file changed