spirv-new does not test fast math mode

See #1483; the `options` variable (and indirectly the `fast_math` argument) in `test_conformance/spirv_new/test_op_fmath.cpp` is not being used.

Start with test_conformance/spirv_new/test_op_fmath.cpp and inspect how the options variable and fast_math argument are passed or consumed. Review issue #1483 for the intended behavior, then verify that the fast math mode is actually exercised by the relevant conformance test.

Written by the indexing model from the issue text.