[libclang][test] Suppress annoying 'LIBCLANG TOOLING ERROR' output

check-all invokes check-clang-python which prints the annoying message:

LIBCLANG TOOLING ERROR: fixed-compilation-database: Error while opening fixed database: No such file or directory
json-compilation-database: Error while opening JSON database: No such file or directory

Let's fix it now with os.dup os.dup2 trick.

llvm-svn: 357562