itk: use vendored googletest (#47687)

external googletest breaks dependents because they end up with
ITK_LIBRARIES set to `GTest::GTest;GTest::Main`, which then end up
literally in a nonsensical link line `-lGTest::GtTest`.

the vendored googletest produces a cmake config file where
`ITKGoogleTest_LIBRARIES` is empty.

var/spack/repos/builtin/packages/itk/package.py

@@ -63,7 +63,6 @@ class Itk(CMakePackage):
     depends_on("eigen")
     depends_on("expat")
     depends_on("fftw-api")
-    depends_on("googletest")
     depends_on("hdf5+cxx+hl")
     depends_on("jpeg")
     depends_on("libpng")
@@ -79,8 +78,11 @@ class Itk(CMakePackage):
     def cmake_args(self):
         use_mkl = self.spec["fftw-api"].name in INTEL_MATH_LIBRARIES
         args = [
+            self.define("BUILD_TESTING", False),
             self.define("BUILD_SHARED_LIBS", True),
             self.define("ITK_USE_SYSTEM_LIBRARIES", True),
+            # https://github.com/InsightSoftwareConsortium/ITK/issues/303
+            self.define("ITK_USE_SYSTEM_GOOGLETEST", False),
             self.define("ITK_USE_MKL", use_mkl),
             self.define_from_variant("Module_ITKReview", "review"),
             self.define_from_variant("Module_RTK", "rtk"),