COMP: Fix BuildSystem tests ensuring GTest with a config file is looked up

Removes the use of NO_DEFAULT_PATH introduced in a709a89b (Cmake updates
for proper generation of iMSTKConfig.cmake) in favor of CONFIG and consistently
use CONFIG in both CMakeLists.txt and iMSTKConfig.

Requiring the GTest config file ensures the configuration succeeds by
preventing the FindGTest module provided by CMake from being used.

Removing the use of NO_DEFAULT_PATH allows to build iMSTK against a system
version of GTest (starting with Ubuntu 21.04 released in April 2021, the
libgtest-dev package includes GTestConfig.cmake)

CMake/iMSTKConfig.cmake.in

@@ -51,7 +51,7 @@ find_dependency( g3log )
 
 # GTest
 if(iMSTK_BUILD_TESTING)
-  find_dependency( GTest )
+  find_dependency( GTest CONFIG )
 endif()
 
 # LibNiFalcon

CMakeLists.txt

@@ -380,7 +380,7 @@ endif()
 # Testing
 #----------------------------------------------------------------------
 if(${PROJECT_NAME}_BUILD_TESTING)
-  find_package( GTest REQUIRED NO_DEFAULT_PATH)
+  find_package( GTest CONFIG REQUIRED )
 endif()
 
 #-----------------------------------------------------------------------------