COMP: Enable ITKGoogleTest whenever BUILD_TESTING is on

[hjmjohnson]

BUILD_TESTING=ON now mandates ITKGoogleTest, instead of relying on each module to list it in TEST_DEPENDS. Any module's test kit may call CreateGoogleTestDriver, and the set of modules with no GoogleTest tests keeps shrinking, so tracking the dependency per module is the wrong lever.

Fixes a configure failure on current main:

$ cmake -DITK_BUILD_DEFAULT_MODULES=OFF -DModule_ITKIONRRD=ON -DBUILD_TESTING=ON ...
CMake Error at CMake/ITKModuleTest.cmake:320 (target_link_libraries):
  Target "ITKIONRRDGTestDriver" links to:
    GTest::gtest
  but the target was not found.
Call Stack (most recent call first):
  Modules/IO/NRRD/test/CMakeLists.txt:425 (creategoogletestdriver)

Found while auditing #6776 / #6777; independent of both.

Scope: seven modules have the same latent omission

72 modules call CreateGoogleTestDriver; these seven do not name ITKGoogleTest in their itk-module.cmake, so any selection that enables one of them without separately pulling in ITKGoogleTest hits the error above:

Modules/Filtering/CurvatureFlow
Modules/Filtering/ImageCompare
Modules/Filtering/DisplacementField
Modules/Segmentation/RegionGrowing
Modules/Nonunit/Review
Modules/IO/NRRD
Modules/IO/IOTransformDCMTK

Adding seven TEST_DEPENDS entries would fix today's instances and leave the next one to be discovered the same way. This PR removes the class instead. The seven entries are left alone; they are harmless where present.

Verification

configuration	before	after	ITKGoogleTest enabled after
minimal + Module_ITKIONRRD=ON + BUILD_TESTING=ON	configure fails	configures	yes
minimal + Module_ITKIONRRD=ON + BUILD_TESTING=OFF	configures	configures	no
default modules + BUILD_TESTING=ON	configures	configures	yes

Enablement checked by the presence of lib/cmake/ITK-6.0/Modules/ITKGoogleTest.cmake in each build tree. The guard is BUILD_TESTING AND NOT DISABLE_MODULE_TESTS, so a testing build that adds no module test directories does not pull the module in.

[blowekamp]

The premise of the first sentence is wrong. Each module needs to have it in there test depends to ensure cmake interfaces are correct.

[hjmjohnson]

Closing. The premise is wrong: TEST_DEPENDS ITKGoogleTest is what makes a module's CMake interface correct, so blanket enablement is not a substitute for the per-module declaration.

The seven modules that call CreateGoogleTestDriver without declaring ITKGoogleTest (CurvatureFlow, ImageCompare, DisplacementField, RegionGrowing, Review, IONRRD, IOTransformDCMTK) should have the declaration added instead.