polish the test structure

Author: zhjwpku

cmake_modules/FindGTestAlt.cmake

@@ -15,6 +15,12 @@
 # specific language governing permissions and limitations
 # under the License.
 
-include(FindGTestAlt)
+fetchcontent_declare(googletest
+                     GIT_REPOSITORY https://github.com/google/googletest.git
+                     GIT_TAG b514bdc898e2951020cbdca1304b75f5950d1f59 # release-1.15.2
+                     FIND_PACKAGE_ARGS
+                     NAMES
+                     GTest)
+fetchcontent_makeavailable(googletest)
 
 add_subdirectory(core)