Remove hard coded toolchain path in /test/CMakeLists.txt

Need a more general path:

```
$ grep -niIr 'toolchain path' .
./test/CMakeLists.txt:240:message(STATUS "GCC toolchain path: " ${GCC_TOOLCHAIN_PREFIX})
./test/CMakeLists.txt:241:message(STATUS "LLVM toolchain path: " ${COMPILER_TOOLCHAIN_PREFIX})
```