Add ewise unary op unit tests for FP_SPECIAL cases

- Implement ewise unary operator tests covering floating-point edge cases
- Covers ABS, FLOOR and CEIL
- Adjust ABS operator implementation to correctly handle sign of zero
  inputs.
- Register new elementwise unary op test file (ewise_unary_tests.cpp) in CMakeLists.

Signed-off-by: Jerry Zhao <Jerry.Zhao@arm.com>
Signed-off-by: Ian Tayler Lessa <Ian.TaylerLessa@arm.com>
Change-Id: I03a74877afd195125dcca03dce6cd0903c8a142e