[CMake] Set sanitizer test C++ library on Linux

[phosek]

Summary

We always want to use the in-tree libc++ for tests.

clang/cmake/caches/Fuchsia-stage2.cmake

if(LINUX_${target}_SYSROOT)
     set(RUNTIMES_${target}_LIBCXXABI_INSTALL_LIBRARY OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_ENABLE_STATIC_ABI_LIBRARY ON CACHE BOOL "")
     set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
     set(RUNTIMES_${target}_LLVM_ENABLE_ASSERTIONS OFF CACHE BOOL "")
     set(RUNTIMES_${target}_SANITIZER_CXX_ABI "libc++" CACHE STRING "")
     set(RUNTIMES_${target}_SANITIZER_CXX_ABI_INTREE ON CACHE BOOL "")
+    set(RUNTIMES_${target}_SANITIZER_TEST_CXX "libc++" CACHE STRING "")
+    set(RUNTIMES_${target}_SANITIZER_TEST_CXX_INTREE ON CACHE BOOL "")
     set(RUNTIMES_${target}_COMPILER_RT_TEST_COMPILER_CFLAGS "--unwindlib=libunwind -static-libgcc" CACHE STRING "")
     set(RUNTIMES_${target}_SANITIZER_COMMON_TEST_TARGET_CFLAGS "--unwindlib=libunwind -static-libgcc" CACHE STRING "")
     set(RUNTIMES_${target}_TSAN_TEST_TARGET_CFLAGS "--unwindlib=libunwind -static-libgcc" CACHE STRING "")
     set(RUNTIMES_${target}_LLVM_TOOLS_DIR "${CMAKE_BINARY_DIR}/bin" CACHE BOOL "")
     set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "")
 
     # Use .build-id link.
     list(APPEND RUNTIME_BUILD_ID_LINK "${target}")