[gtest][CMake] Compile the unified source gtest-all.cc

https://bugs.webkit.org/show_bug.cgi?id=259090

Reviewed by Yusuke Suzuki.

clang-cl reported the following warning for Windows port:
> ThirdParty\gtest\src\gtest-filepath.cc(68,12): warning: unused variable 'kAlternatePathSeparatorString' [-Wunused-const-variable]

clang-cl doesn't report this warning for the upstream google-test
because it's using gtest-all.cc. Let's use it in WebKit.

* Source/ThirdParty/gtest/CMakeLists.txt:

Canonical link: https://commits.webkit.org/266025@main

Source/ThirdParty/gtest/CMakeLists.txt

@@ -10,15 +10,8 @@ set(GTEST_INCLUDE_DIRECTORIES
 )
 
 set(GTEST_SOURCES
-    ${GTEST_DIR}/src/gtest.cc
-    ${GTEST_DIR}/src/gtest-death-test.cc
-    ${GTEST_DIR}/src/gtest-filepath.cc
+    ${GTEST_DIR}/src/gtest-all.cc
     ${GTEST_DIR}/src/gtest_main.cc
-    ${GTEST_DIR}/src/gtest-matchers.cc
-    ${GTEST_DIR}/src/gtest-port.cc
-    ${GTEST_DIR}/src/gtest-printers.cc
-    ${GTEST_DIR}/src/gtest-test-part.cc
-    ${GTEST_DIR}/src/gtest-typed-test.cc
 )
 
 set(gtest_DEFINITIONS GTEST_HAS_RTTI=0)