Allow CL_HALF_FLOAT denorm flushing for write tests (#452) (#453)

* Allow CL_HALF_FLOAT denorm flushing for write tests (#452)

* On mismatch, add relaxation when denormal half result is expected
* Refactor to use common validation function
* Clean up some diagnostics

* Fix review comments

- use cl_half
- remove extraneous casts
- replace literals with sizeof()

* Document rollover trick for IsHalfSubnormal