COMP: Fix the test link when ITK is built with wrapping enabled

[hjmjohnson]

Configuring against an ITK built with ITK_WRAP_PYTHON=ON fails in test/CMakeLists.txt, and the link it sets up is rejected by Apple's linker. Two CMake-only fixes; no C++ changes.

1. Plain vs keyword target_link_libraries

ITK creates the driver and links it with the keyword signature
(ITKModuleTest.cmake), so the plain call in impact_link_python_embed stops
configure:

CMake Error at test/CMakeLists.txt:51 (target_link_libraries):
  The keyword signature for target_link_libraries has already been used with
  the target "ImpactTestDriver".  All uses of target_link_libraries with a
  target must be either all-keyword or all-plain.
   * .../ITK/CMake/ITKModuleTest.cmake:84 (target_link_libraries)

ImpactGTestDriver is the mirror image: its own link is plain, so it fails as
soon as the function adds a keyword call. That block runs even with
Module_ITKGoogleTest=OFF, because ITK still exports a GTest::GTest target.

Both now use PRIVATE.

2. --no-as-needed on linkers that reject it

ld: unknown option: --no-as-needed

The flag keeps GNU ld from dropping libpython, which Apple's linker never does.
It is now passed only where it is understood.

Why this is not visible in CI today

impact_link_python_embed returns early unless ITK_WRAP_PYTHON is set, so the
failure needs an ITK built with wrapping enabled.

Verified in a downstream build matrix, macOS arm64, LibTorch from the torch
Python package:

ITK	wrapping	module source	result
main (6.0.0)	ON	unmodified	configure fails
main (6.0.0)	ON	with this PR	builds, 51/51 tests pass
release-5.4	OFF	unmodified	builds, 51/51 tests pass

The 5.4 column is why this reads as a wrapping-enabled issue rather than an
ITK 6 one.

https://claude.ai/code/session_01S5zzoU6yyUwdLEvjM2s6iS

[vboussot]

Thanks!