[libcxx] [test] Quote the path to the python interpreter

This should allow running tests with the interpreter in some of the
default paths where Python for Windows might be installed.

Differential Revision: https://reviews.llvm.org/D97369

libcxx/test/CMakeLists.txt

@@ -82,7 +82,7 @@ endif()
 
 set(LIBCXX_TARGET_INFO "libcxx.test.target_info.LocalTI" CACHE STRING
     "TargetInfo to use when setting up test environment.")
-set(LIBCXX_EXECUTOR "${Python3_EXECUTABLE} ${CMAKE_CURRENT_LIST_DIR}/../utils/run.py" CACHE STRING
+set(LIBCXX_EXECUTOR "\\\"${Python3_EXECUTABLE}\\\" ${CMAKE_CURRENT_LIST_DIR}/../utils/run.py" CACHE STRING
     "Executor to use when running tests.")
 
 set(AUTO_GEN_COMMENT "## Autogenerated by libcxx configuration.\n# Do not edit!")