Tests: Require git to enable RunCMake.CMP0150 test

The test added by commit 550f63447d (ExternalProject/FetchContent:
Support relative remote URLs, 2023-04-21, v3.27.0-rc1~156^2) covers
Git-specific functionality, and requires a `git` tool to be available.

Tests/RunCMake/CMakeLists.txt

@@ -160,7 +160,10 @@ endif()
 add_RunCMake_test(CMP0132)
 add_RunCMake_test(CMP0135)
 add_RunCMake_test(CMP0139)
-add_RunCMake_test(CMP0150)
+
+if(GIT_EXECUTABLE)
+  add_RunCMake_test(CMP0150)
+endif()
 
 # The test for Policy 65 requires the use of the
 # CMAKE_SHARED_LIBRARY_LINK_CXX_FLAGS variable, which both the VS and Xcode