Fix `unsafe_trunc` test for `NaN16` (#56630)
The return value of the LLVM instruction `fptosi`
(https://llvm.org/docs/LangRef.html#fptosi-to-instruction) does not
guarantee that the truncation of `NaN` is 0, so we relax the test to
only check that the output has the expected type.
Fix #56582.
