[XRay] Run tests inside bootstrapping build (#168378)

COMPILER_RT_STANDALONE_BUILD is set when doing a bootstrapping build
through LLVM_ENABLE_RUNTIMES with the CMake source directory being in
llvm/. This patch changes the XRay tests to also detect that we have
LLVM sources and the llvm-xray tool if we are in a bootstrapping build
through the use of the LLVM_TREE_AVAILABLE variable which is set in
runtimes/CMakeLists.txt.

GitOrigin-RevId: c6f501d479e82185a5906096b758480e43f2edcc