Improve UnitCL pow testing.

[hvdijk]

Overview

Improve UnitCL pow testing.

Reason for change

We single out the value 0x39f6 in our implementation of log2_extended_precision, so in tests that use that function, we should ensure that that value is always tested.

Description of change

This adds an overridable GetEdgeCases() function, defaulting to the InputGenerator::half_edge_cases we previously used, but overridden for HalfMathBuiltinsPow to additionally add 0x39f6 as an edge case.

Tested by locally reverting commit 90cf2fa and verifying that UnitCL reliably picks up the error.

Anything else we should know?

If there's any other relevant information we should know that may help us in
understanding and verifying your patch, please include it here.

Checklist

• Read and follow the project Code of Conduct.
• Make sure the project builds successfully with your changes.
• Run relevant testing locally to avoid regressions.
• Run clang-format-16 (the most
  recent version available through pip) on all modified code.